Well honestly I think you should learn to like his fashion choices because he is your man and you should respect his choices. BUT if his fashion choices are awful I think you should just tell him that you don't like it because if you are in a good relationship I think he will understand and respect your opinion.
I knew someone that was quite a catch that took issue with his girlfriend for suggesting that she liked when he wore such-and-such more than his usual such-and-such and ... well that was the end of the two of them as an item. If a man is young and not ready to commit, he will bail for the slightest annoyance. I really don't think this girl brought it up in a bad way either. I wish there was a good way to do what you are trying to do. Best of luck!
