Take it to the vet. Isolate it from any other hamsters in the cage with it. It could be a rash form of a virus although vets offices for hamsters are rare and costly. What I would do is wash it with dawn dish soap and clean the whole cage. It also could be mites. What they are are tiny little bug that irritate the skin. It is not contagious to humans but you must wash your hands after each time you handle it. What I did when one of my patients had it I suggested to use ADAMS FLEA AND TICK MIST. Go onto ADAMS FLEA CONTROL.COM it should still have it or they might sell it at a local pet store. I hope your little guy feels better soon.