+1 The size is likely to be 190/50ZR17. But there's more to it than that. He may prefer a certain brand. Beyond that, is he after maximum grip or does he want something that won't wear out so quickly. Can you find out what he has right now? Note the sidewall markings and post them here.
It sounds like you don't ride. I'm not trying to be condescending but it's sort of like buying a dress. There's a lot more to it than size. Buy it from somewhere that he can take it back to exchange.