First all there is a rule of thumb that goes 'you can't teach anyone anything, you can only give them reasons to one to learn'. That is true of everything. The second rule is that the method of learning has to be -relevant- to them. So your task is to find reasons for your boyfriend to want to 'grow up' as it were.
It is also the very essence of love to find ways of helping your partner (or anyone) to find ways to grow and develop as a human being. Try new experiences and activities, as experiences determine who we become throughout our life. And so on.
I believe that ladies can help their lover. I don't like when snobs or society looks down and call someone a "boy" and that they are grown up and whatnot. I find that to be snobby behavior. If anything that shows how miserable some puritans can be and cranky. A lady should not be judgment but help her love become a grown up. Teenagers we can be stupid, but we shouldn't be judged for it. Acting like a boy ain't the worst thing in the world. It's not like you're married to a murderer, or a child molester. So your lover is childish. OK, can be fixed.