How do I make my parents trust the guy I love more?

Alright, I have someone who I love very much, and has not made the best choices. Although since we've been going out he has quit smoking and has been improving his life step by step. He says he will improve himself as much as possible to make this relationship work. (I had told him before I do not tolerate smoking or anything involving not thinking!) Were both not fully grown yet, as in were both in high school. He comes from a horrible family, but he's been approving very much with my support. I want to show my father this before I reveal our secret love for me and him. Although my father is very strict and does not approve of him at all at the moment..He even doesn't let me stay in the same garage with him without him there or something. What in the world should I do to make my father become more comfortable with him? Help me please? (: