Echoing the others that I wouldn't try to change him, but maybe encourage him to think about clothes as part of the process of getting ready for special occasions, IF the way he dresses stands out at the event and might possibly seem unintentionally disrespectful to the hosts. For example, dinner party with parents/an older generation yes, dinner party with casual friends of your age, maybe don't worry about it.
I'm kind of laughing because I just realized I know someone who dresses the exact same way down to the bare feet indoors when he's hanging out at home or with friends or family (and is a little older) but is very well educated and respected in his field, he's just focused on other things and that's how all his friends dressed in college, so why would he change? He wears snazzy outfits when he needs to for work or weddings, otherwise he's good.
Otherwise, positive reinforcement -- buy clothing as gifts for him, tell him he looks great in certain colors, styles, etc. and see if likes the gifts and compliments, or not. If not, I'd drop it.