Sounds like you realized you were no longer interested in this young man as a boyfriend. That doesn't mean he gets to keep your heart hostage because you are his first gf. You need to honor your feelings and act on the fact that you do not like him the same as he likes you. I'd let him know in a nice way that you would like him for a friend.

That way you both are free to persue other people for dating.

Wishing you luck.