Slowly and in an environment where he's comfortable.
What? That's not enough to help, you say? Okay, my advice is to think about what those terms mean to you. He's not going to know what a "little" is or what's expected of a caregiver, at least not unless he's been doing a bunch of googling on the side. You can use the terms, but it's much better to think about an explanation that removes them. What is being a little to you? Is it about the activities? About being treated a certain age? About being free to act out a certain aspect of yourself? Do you have certain things you really want him to do? Ways you want him to act or words you want him to say?
As an example, which of these makes more sense to you?
OPTION 1: "sometimes, I just want to feel a lot younger than I am. Not just wear diapers, but to really act like I'm 3 years old and not potty trained. And I'd like it a lot if you could be part of that as an adult, so that sometimes I can act like I'm little and you can take care of me."
OPTION 2: "sometimes, I really want to do little kid things. You already know I wear diapers, but more than that I want to drink out of a bottle, wear baby clothes, play with toys, color, that kind of thing. I'd really like it if you could participate too. You could read to me, or dress me, or feed me even if that's something you would enjoy doing."
OPTION 3: "There's a part of me that like to be a really young child, and I want to let it out sometimes. When I'm in that space, I like to act like I'm 3 years old, and I feel very vulnerable and even start to see the world a bit differently. It's a lot of fun when done safely, and I want you to be a part of that by acting as the role of a father when I'm in that space."