How do you tell a guy that he needs to rethink his dress sense?

My advice would be to not be shallow. Plain and simple. Put yourself on the reverse side. Would you want to be told that someone didn't like something about you? If ONLY you'd change then they'd be madly in love with you??? Look, if you like this guy, then like him for who he is, not what you think he can become. You and i can't change people, but what we can change is how we look at them. If you really care about this guy what you'll do is just deal with it. If you feel like you can't, then let him go. I wish you all the best!