Well the kids i raised for the most part were pretty good, and i'm pretty lax too, but when they run away that isnt good under any circumstance...understandable but the A of the AB needs to be in charge and go through the whole thing...
I've dealt with a cousin and sexual misconduct from his uncle...to the usual i broke something...
I always tried to be the parent that didnt punish or take things away for nearly anything, and talk to them as to why (if it was) it was wrong and what may have been a better option.
As for the exact situation, i'd let him alone for awhile, maybe 10-20mins later knock on his door if he didn't surface for air yet
Ask him why did he run, and explore what exactly was going on, and not jump to the ABDL choice...
Could have been a dare/bet, or someone told him to try it, or a plethora of things outside our ABDL niche kink.
whatever has happened, i always try to get as full an explanation as possible, age biased of course!
So if he said that he wanted to try wearing and/or using a diaper, i'd ask why, and explore well in advance to expecting he is exploring a fetish.
If he was wanting to wear for pleasure and/or security, would explain to him that it's ok, this isnt something that will physically harm you and if you wish to explore that is ok, and he doesnt need to fear or keep it from me. I'd much rather know whats going on then to try to correct something that isnt a direct threat to them....i've had to deal with the pot, and the sneaking a beer and coke and meth...I may live in the sticks but i'm 1hr out of a top 10 metropolitan area.
Now, i'm also going to watch that i don't push him into it either, there is a fine line from tolerance to encouragement....
Not going to offer the give him diapers, or advice on things to do...now if he comes to me and asks for help specifically that is a different thing, and i've had the ask help with underwear types, to shaving, to all sorts of things and i do help and they always knew they could ask just about anything, even about things like pot, hard drugs, or alcohol...
Whilst technically being illegal, on special occasions they would have a few friends over and they could have some alcohol before htey were 21! Yep!
Everyone there was forbidden to walk off the property and i collected all keys to cars, and everyone must have thier parents talk to me in person about this in advance, well in advance.
I truly don't think age limits do any good, some people are ok to drink at 16 others aren't ready yet at 61....and it just forces the young men and women to go places not safe and drive to and from there after drinking...NOT GOOD PLAN!
Anyhow, I know i'm rambling tonight...
But, my take is not to jump to conclusions, one way or the other, get the story, and talk to them about it, and see not just why this small thing, but the larger picture, if they said they like the security of it, as them if they feel unsecure and why and about what things...It may be nothing big, it may be that thay are afraid of something real or inferred, right or wrong...perhaps they are afraid that you and the so are breaking up, or that they are coming between you two...and want to go back to a time when they were just a baby and didn't impinge on time or resources so much.\
One boy was afraid to ask for money i found out, wasnt because they though they were going to be denied or taking too much, but rather wasn't totally clear on money, as with the credit/debit cards, and i still used a lot of cash too, not to mention putting things on account...he was afraid that he wouldn't know what to do/exchange/get back from a transaction...This was a normal learning curve for a young boy, but being on my farm, and most things come in from the truck (on-line) and going to a buying club for organic foods we couldn't barter local...i could see where he didn't get alot of exposure to the whole process...especially tax...that one was tough, i don't get much in the way of easily viable services at home...I used to grade the road myself out to the main highway (before more people in the area) yes it's dirt...and of course everything else I pay for on the property, and pay taxes to boot...but that is another topic all together...
Moral of this long winded story.
No matter what, don't jump to conclusions, get the whole story, and then talk through it again to make sure...then give helpful suggestions.
It's not easy, it hard to do, and i've jumped the gun myself, but i try to do that...as of now i'm kidless (as is my ex) getting old, just me an an assistant rummaging around my house most days, with some times others stop in for a bit.