If you don’t want him to act like a young child, stop treating him like a young child. That’s for starters.
Since you haven’t told me exactly how he’s acting like a young child, I can tell you that if he has a great job, makes sure to take you out and give you attention like a great boyfriend, but wants you to tie his shoelaces, I’d just buy him loafers to replace the tie shoes.
In other words, if most of what he does is very mature and responsible, but he’s eccentric, then you can work with someone like that because on the proverbial score sheet, his good column outweighs his bad column. But if pulls your hair to get your attention, expects you to treat him to dinner, movies and all your date activities, and has a potty mouth, then why date him?? Don’t you want a man — not a child?