Alot of people dont realize that men can get yeast infections too. They dont present like they do in women, so they're not as easy to diagnose. That could be the problem. Get him some OTC yeast cream and see if that helps. The hard part is convincing him to try it ;)
He could be allergic to something used 'down there'. Like certain types of lube or certain types of condoms. Or it could be a yeast infection. Or he could just be naturally itchy =] You should try to convince him to see a doctor, and promise him that you won't tell anyone, lol.