I need Girls help with my bf and his small one what kind of underwear should he wear?

ok ladies my boyfriend has a 3 and a half inch in his pants when he's hard and any kind of men's underwear just don't look right on him, what other then men's underwear do you think I should put him in there has been a number of suggestions thrown at me like speedos, little kids underwear, nappys, panties but what kind, hmmm I just dont know what do you girls think be honest.