He looks small enough for it to just be molting and his hair could come in normally. Perhaps he has some rex in him(that can make the hair curly or if he's a double rex, the hair could molt out and regrow and sometimes look hairless).

It could be things like mites or skin issues too. How long have you had him, what sort of bedding is he on, what food does he eat? These things may also help answer the question, especially when some of the truly brillient rat people on here chime in.