Most Helpful Guy
Some people are naturally good with fashion, others are not. It's just the way we are born and how we see colors and designs.
I too am a poor dresser. I used to wear tattered t-shirts, and tucked them into my track pants. Yeah...you heard me right.
You might try suggesting (or just ordering him) a subscription to a magazine like GQ, or start buying him nicer clothes. He will not have the eye to be able to select nicer clothing so this will probably require some push or initiative by you. Skip the suit and tie and go for collared shirts and sweaters that fit nicely and comfortably.
He basically needs to be tutored on what looks nice and what types of colors/patterns make a man look good. He's spent his whole life paying no attention to fashion or style. There is little to no capacity for him to learn it on his own now. I would avoid blaming him about this problem. He cannot help it. It's like telling a person who's bad at math to suddenly be awesome at complex calculus and derivatives. It just doesn't happen without a lot of tutoring.
I suspect your boyfriend has a very laid back job (i.e. mechanic, tradesperson, IT), and not a business-type job. If he is not expected to dress well at work, he pretty much has zero reason to dress nicely...period.