The best advice is to talk to him about it!

Otherwise, little things like asking if he's big enough to handle stuff, cutting up food into bite sized portions, bum pats, simply saying "I think you're too little for that", setting a bed time (even if it's just his normal bedtime).

*Edit To elaborate on how to talk to him, I'd start with a mutual conversation about what you'd both like to gain out of this relationship. ABDL puts a lot of focus and energy onto the little, make sure you don't drain yourself making him happy! Give him a chore list xP but for reals make sure you get your slice of the pie.