i know it sounds dumb but he may not be able talk to you. i dont know how to talk to my gf when shes around me because i fumble my words and say things that i dont mean or that accidently upset her so i slowly talk less and less. away from her i can talk to my friends day and night about her and all...but in the end you will just have to talk to him and dont pressure too hard
Sit down and have a talk with him. Only he knows why be speaks about you and not to you. Quite honestly, if he can't talk TO you, I don't think it's such a good thing to be with him. Friendship is probably the most important ba<x>se in a relationship and one of the factors of it is communication. =]
He has told you once, and feels that was enough.
If he dosen't talk to you, but talks 'about' you, you should have dumped him before the first month went by. But since you obviously didn't ( for whatever reason ), after 5 years don't expect him to change, he's not going to. As to the "Why" part of your question, would you believe he's probably an insensitive moron who dosen't really care how 'you' feel, it's all about 'him'.
If his 18 it is doubtful he wants to marry you right now; and most boys that age are not trully thinking of marriage in their short term future.. Enjoy being young, have fun w your bf, but don't try to rush into adult relationships like marriage.