Is your hamster a syrian or a dwarf? How old is he?
If he is a dwarf, the patch on his belly is likely the scent gland. Male dwarfs have a scent gland on their belly and sometimes it just gets dirty, you can either use a paper towel with warm water to wipe it, or typically they will clean it themselves.
If it is a syrian, their scent glands are on either side of his hips, so it could just be pee or something. Has he been sleeping a lot in his pee?
If it is a bald patch and his skin is brownish green then it could be a fungus of some sort.
It is hard to tell without a picture though. There is a hamster forum called Hamster Hideout, if you can't get enough answers here, you could always try posting on a forum specifically for hamsters. I am on that forum and they are really helpful over there.