Not sure if it'll help, and I'm a bit late to the party, but thought I might give my experience as someone who told my parents as a teenager (this might get a little long). There are a lot of things I've learned since then, and it might be good to share. The first question of course is whether or not to get involved. It's a personal decision, and you'll probably know what feels right, but talking about the whole diaper thing has been something that only made my bond with my mom stronger thanks to how understanding and supportive she has been, which I think is key. I still can't help but laugh a little with how nervous I was though at the time, since because of how I was acting she was terrified I was about to tell her I was doing drugs or something like that. In her words, "Thank god it's just diapers." After that, I asked if we could do some therapy which was really nice in giving us a better open environment to talk.
The first thing to note though is that there's no avoiding the fact that it's embarrassing. Like it or not this is a taboo, but it does get easier to talk about with time. One of the things therapy has taught me to be active instead of avoidant. Facing problems and fear instead of ignoring it all is the best way through, even though it's hard and often very uncomfortable. While I understand where others are coming from in wanting to not talk about it, it's also something that perpetuates problems instead of resolves them, at least from a psychological perspective. You're doing the right thing by having a conversation as a parent. The conversation itself though is a matter of love and understanding and needs to be framed as much.
To start, something important that can help is priming. Don't have the conversation right away, or if you do, try to frame the conversation about what it is you want to talk about without being too direct. It can make the listener more receptive to what you have to say and help avoid any surprises. This can look like a lot of different things from "Hey, if you have some time this evening there's something I wanted to talk with you about," "Hey, there's something I wanted to talk about. It's a little private so just let me know when might be a good time," to, "There's something I wanted to bring up, but it can be a little embarrassing/uncomfortable. Is now a good time or would you prefer to talk about it later?" or "There's been something on my mind and I was hoping to talk about it. I haven't known how to bring it up, so is it okay if we have a conversation?"
Those are just some examples to give a better idea of what I'm talking about, but they help in both easing into the conversation and give them some choice in if they're comfortable with the conversation or not, that they have the option to say no. The conversation is something that should happen, but they need to be ready to have it too. Try to have a kind and casual voice when talking about it and avoid making it seem like something serious. It's embarrassing, but to talk about this stuff properly we need to create open spaces where there's a general understanding that, yes, this is something that's okay to talk about and that these things are okay. The main thing is just to avoid starting the conversation off with "I found some of my diapers were missing, do you know anything about that?" It's not the worst way to start, but it immediately puts the person you're talking to into the hot seat and an uncomfortable position, more so than if the conversation is eased into.
Just one small note, if you do prime the conversation, do avoid putting too much time between when you bring up the conversation and when you have it. Leaving a lot of time between priming and the talk itself can lead to unnecessary rumination and stress, which isn't a good way to start the talk off either. But, after initiating the conversation there are a few important things that can be done to make it go more smoothly, as well as some general pointers.
First off, being able to diffuse a situation is very important, as well as knowing how to frame the conversation. One big piece of advice is addressing the bad and naming emotions. Talk about emotions first. If they seem nervous, talk about that first and don't let it sit. Hard conversations need to be done with a positive and open mindset, and being nervous, angry, overwhelmed, or embarrassed can interfere with that and make people combative and resistant. Focus on them first and build empathy. One of the best ways to handle negative emotions is to address them and get them out in the open. One common tactic is naming the emotion. Calling feelings out is something that helps make them easier to handle, and can help us process them so they're less all-consuming or override reason. To do this, start with "It seems like..." "It looks like..." "It sounds like..." or some variation on that idea. Then add the emotion at the end to call the feeling out: "It seems like you're feeling uncomfortable. Can you tell me what's going on first?" Just try to avoid directly telling or inferring how they feel like saying "You're mad, and..." as that can put them in a defensive position. It can seem a little artificial, and there can be some pushback, but addressing emotions can help diffuse them and help make way for a more productive and open conversation in the long run.
With this, above all, it's important that they feel heard, seen, and safe. This kind of conversation is one built on being open and wanting to understand, and it's important to avoid simply acting in their best interest. Instead, be sure to listen to them and ask what exactly their needs are and what they want. For me, the hardest part was not wanting to be seen or treated differently despite the whole regression thing, and that the best way that my parents could help was to let me handle things. It was a conversation that only happened after my mom took the initiative and bought a case of pull-up depends at Costco. It took a while to say that wasn't really what I was looking for even though I knew she was just trying to help. And this is the important part, to focus of the conversation in an open-ended way that encourages feedback, like saying "I wanted to talk because I noticed some diapers were missing and I wanted to ask about it." It's important to phrase this as an inquiry because it gives them an out to say no if they don't want to have that kind of conversation or don't have enough trust to talk about it at the moment. If the answer is indeed no, it's important to respect that and let the topic go. Just be sure to let them know if anything happens, changes, or comes up that you're always there to support them and help, whatever that looks like. At the very least you're modeling good communication and conflict resolution skills, which is important in its own right. And if they are ever ready to talk they know the door's always open and they can come to you about it.
Otherwise, if they say yes, the best advice I could ever give is to listen, be curious, and inquire. They will let you know what you need. There are a few good points to talk about though, primarily the why, what they need, and establishing healthy boundaries. For the why, there are tons of reasons from curiosity to fetish to incontinence to regression or big mixes of a lot of things. Understanding the why to the want is important and can help move the conversation forward, allow them to feel heard, and give you a better understanding of their situation. It's also something that will heavily influence what boundaries are put in place and how to best help. Asking "Alright, is it okay if you tell me why?" can help. You can also just ask point blank, "So why did you do that?", but I find asking permission for a question helps give some security and helps the other person know that it's okay to say no. There's a lot of security that comes from being able to say no so it's something that can be pretty important to keep in mind to create a safe, non-judgmental environment. Confidentiality also does the same, knowing that nothing they say leaves that room without their permission, which is good to let them know.
Next up is knowing needs. Once you know why, you can get into what they need from you. This can be emotional, or it can often be material. Personally, this was something I wanted to keep private. Outside of needing space for regression, I also needed the ability to take care of things myself and didn't want to get my parents involved. I bought my own diapers, pacifiers, onesies, and those sorts of purchases. Granted, I do have some regrets though as I think I asked for more independence than I could realistically handle. It lead to my room smelling like piss, water stains on the floor or in my closet, and a whole host of things I didn't know how to manage. Luckily now I've learned better ways to handle things like buying a diaper pail, knowing proper wearing and disposal, and being way more responsible with my hygiene. But it was hard in the beginning and is something to watch out for. If you see them struggling, needing help, or otherwise mishandling the situation, do step in so they can learn how to do things right. The more they feel they need to hide the more problems it's probably going to create, and the last thing anyone wants is a month-old trash bag of soiled diapers in the closet.
The big thing though is often who buys what. Either they can tell you what it is they want, or you can give them an allowance. But, what's important is to ask. My mom, while well intended, has gotten me a lot of little stuff from onesies to pull-ups that weren't what I was looking for. Though the more we've talked the better things have gotten, and I adore the Bluey shirt she got me. Also, getting a job helped immensely as I finally felt like I could be more responsible for myself, take care of my own needs, and be more independent, which could be another solution. Or, maybe they can agree to do optional chores around the house for some extra cash if need be. It can be a big boost to start having that trust and independence as a teenager, at least speaking from personal experience, and was part of the emotional needs that I expressed to my mom, but everyone is different. As long as they can communicate what it is they need from you, that's what's important.
Lastly, something to talk about alongside needs are boundaries, rules, and expectations. A core value I've kept for some time now is to make sure that my behavior doesn't negatively impact others. This is often tied up with respect and limits, but it's the main rule of thumb I use for where to draw lines. For example, one of the first boundaries I established with my parents was time and place rules. It was okay to wear diapers at home, but they needed to be worn just like underwear, which I think is a good ground rule. Beyond that, in the case of AB or DL tendencies, there are other aspects to talk about like how much to keep private. There's no need to be overly involved in anyone's sex life, but it's not always sexual, and it's hard to know without asking. Part of my needs were very AB and regression-related so we came to the agreement that it was okay around the house, but needed to be put away when guests came. This is the boundary that has actually changed the most as we've all become more comfortable. Now things like stuffed animals, fun blankets, and comfort items stay out in the open, and I keep my diapers and changing supplies organized on a small shelf next to the pail at the end of my bed. It's something that is accepted and treated as kinda normal. I'm still me, just with another side of myself showing. Which, this is to say that it's good to revisit and re-establish boundaries every so often, because changing them can be a very good thing. Needs can change, and it's important to accommodate that. That goes for both of you too. Remember to make your needs and boundaries known as well.
In negotiating, it's less of a matter of giving permission to do something and more just figuring out how to make things work, or how to make things okay on both sides. So long as that's the general approach I don't think there are too many ways to go wrong. Mainly it's important to be cooperative and give them the space or things they need for whatever they're going through. It sounds like you're coming from the right place though so I wouldn't be too worried so long as you have an open position.
Some last few tricks when it comes to going about asking questions, a lot of understanding comes from getting more information and clarification. An easy trick for building up empathy is repeating/rephrasing what they've said and asking if it's correct. It's part of mirroring, which is a technique that helps avoid judgment. This involves repeating certain words back to them in a way that asks for clarification. For example, say they offered "I've been feeling weird lately and I don't know what to do," it could be a good option to respond with "Feeling weird?" It's simple, but it can work pretty well. However, something I picked up from my therapist are more prompting questions. These are remarks like, "Tell me more about that," "Can you elaborate?" "Can you describe how you're feeling a little more?" "If you can I'm curious to hear more about that," "Can you give me an example?" Questions like that can be SUPER helpful, let them know you're there to listen, and avoid making judgment statements. The idea is mostly to help them walk through their own feelings, as well as gain a better understanding and make it easier to empathize.
Along with avoiding judgment statements, using "I feel-" statements can be a great way to approach personal perspective. They're used in counseling and often prove very helpful in making emotions more approachable and subjective. It's actually a great thing to teach to kids to help with learning conflict resolution, saying "When you do this, I feel that" instead of making personal attacks like "you hurt me, how could you do that." It can be a good response, phrasing the situation in terms of emotions rather than just actions. "When you take things behind my back I feel like you don't trust me, or that I can't be trusted, which isn't what I want. What can I do to help?" might be an example in this case. Overall, the framework for "I feel" statements is good to have on hand when expressing personal opinions or feelings, especially regarding the behaviors of others.
Another tip is to when possible avoid "you" and negative statements. This goes back to naming emotions and not telling others how they feel, but "you" is a tricky word to use as it can be very personal and easily make someone feel attacked. This is mostly regarding comments like "You're being unreasonable," "You're sick," "You have to work with me here," "You're wrong," "How could you say that," "You're not listening," and remarks of that nature. It's not to say they're common, but being aware is important, especially if it means needing to self-correct, like "I'm sorry, I shouldn't have said that." I doubt anything like that would even come up, but it's always possible so I thought I'd include it. In addition, avoiding negative remarks is a small way to keep a positive frame and avoid equating things to something they're not. For example, something I try to do when talking about regression is instead of saying it's harmless, that it's helpful. It's a small shift, but something that avoids making associations that I don't want to make. It's a good technique that's used when dealing with the news and press actually, to keep the focus on what things are rather than what they aren't. It's a subtle language shift that can do some good. Going from "I'm not here to hurt you," or "I'm not mad," to "I'm here to help, just tell me how," or "It's okay," is a nice positive shift. It's not something that's going to make or break a conversation, and sometimes you gotta use a negative, but it's a helpful trick to remember sometimes to keep conversations more positive.
There are a few other tips and tricks but they're more minor. These are the real core comments I feel are important for these types of conversations. Opening up can be hard, but with a little effort and understanding it can be a great opportunity to help them navigate a weird part of their lives and even establish trust. The biggest point is to bring acceptance, love, and support. Often that can be the hardest thing to find when it comes to anything ABDL or regression-related. Diapers can be pretty positive, and it's important to guide them in the right direction to be responsible. In the end, everyone's different, but the ultimate goal would be to make sure that diapers can be positive and add to their quality of life, and that they learn to avoid escapism or acting on feelings without thinking. With all of this, healthy regulation is key of course, and it's important to provide an opportunity to learn about that where many unfortunately don't. I was lucky enough to have help and am far better off because of it, but it's not always the case. Teaching how to be safe is the best way to avoid unnecessary struggle or things going wrong. I know these aren't usually conversations we want to have, because these things tend to be very personal, but they're important to help grow, and build trust, understanding, and relationships. No doubt I imagine you're plenty aware of all this, and I'm probably preaching to the choir, but I feel like it needs to be said anyway.
I wish you the best of luck in your relationship and hope everything can work out well. Also, I'm no expert so I'd say take what I say here with a grain of salt (I'm also making this while tired so hopefully my writing is okay). While being open with my parents was hard, it has been incredibly helpful and rewarding in the long run. But, my situation is my own. I can only hope for the same for you and your son, and that he can be set up for success moving forward. Good luck