Anything that has the right fit. This means polos, t-shirts, button downs, or sweaters that aren't baggy with wife beaters underneath. Same with jeans, keep fit in mind, a regular to snug fit and they don't hang down past your boxers. Also stick with dark wash denim.
And it's not about the clothes you wear really. It's more about the confidence you wear.
Something that shows off his body and that fits right. If his clothes are too baggy then it looks nasty, if he's wearing really tight clothes and he's skinny it just looks gay, if he's fat it looks gross.