Hi! I married one of these. I only recently (as in: in the past three weeks) got him to get rid of his hideous pink-and-green tie-dye tshirt from his middle school chess club. Yes it still fit, yes he still wore it.

But we've hammered it out over the years that when it comes to formal events I basically dress him. Not saying it'd work for every couple, but it works for us. And you know what? Over the years, he's started to like the things I pick out for him. He's got a gorgeous mane of red hair and when he wears it down with the fitted gray peacoat I got him, he gets compliments left and right. And it's turned from 'oh not my grungy t-shirts :(' to him having a sense of pride and reaching for those items with confidence that when he puts them on, he looks good. Damn good. I'd say 95% of the time I'm there with the handsomest man in the room, but I'm biased. ;)

So... my advice is to honestly just ask if you can take him shopping and pick him out clothing appropriate for formal events and fancy dinners. If he's anything like my husband, he knows that there's a different standard of dress for more formal events, it's just that actually learning what looks good and then going and buying things without guidance is too much hassle when he could be schooling his coworkers at chess instead.