You've gotten some good responses already. New home sneezes are fairly common, as the rats adjust to their new environment. Taking him to the vet would be a good idea anyway, just to make sure he's all right and also to get acquainted with the vet. I also second Vlad's concern about keeping him by himself. It's even more important when it is a hairless rat, because they need the extra warmth from their cagemates' bodies and fur to keep warm and happy.