A small scaly patch of skin could be caused by many things. The most common causes are mites, allergies and fungal infections but if the patch isn’t spreading then this seems less likely. It could also be his scent gland which is located on the side of the hip. Hamsters rub these glands against territory to mark it as there own, causing them to loose fur. The dampness of the area often leads to irritation which they then scratch, aggravating the problem. There are other more serious illnesses that you should keep a watch for like lymphomas (skin will go thick, lumpy, crusty and sometimes ulcerous). In these cases a visit to your vet is a neccessity.