Thank you so much, @Tart Cherry Jam. I’ll definitely take a look at Reddit, I really appreciate the suggestion.
After doing some searching online, I actually came across an article on WikiHow that talks about this topic: wikihow.com/React-when-Your-Teenager-Is-Wearing-Diapers
It helped me feel a little less alone.

More and more, I’m starting to feel that this might be tied to some kind of anxiety he’s carrying, that in certain situations, wearing a diaper helps him feel calmer and safer.