What can I do with an immature boy ?

Well thats why the american stereotype is 14 year old girls dating 18 year old guys this is because thats when the maturity levels balance each other normally. Guys mature slower and later than girls thats why we generally think the majority of them are total idiots, there isnt much you can do for your boy, just tell him when you think he’s acting like a bit of a child so he’s aware of what bothers you and he can try to work on it. If you really love him and you cant beaer to let him go you just have to put up with him being a bit immature– but really try to show him what areas he needs to grow up about, if it doesnt work he wont take your advice or just isnt capable of being more mature yet, there’s really no way around it you’d have to leave him… but for now just wait it out, he’ll start to grow up more