There could be many answers to this question, and to be sure you may want to take him to your local vet. First, you may want to think back and see if there is anything that you may have changed in the cats surroundings, if so, your cat may be allergic to some ingredient involved in this change. If not, your cat may have some other allergies going on with his skin and may need medication. Another alternative may be for you to check for fleas or ticks. This may sound weird but you may want to check and make sure that your he is not a she and she may be going through her monthly cycle. Sometimes it is hard to tell if a cat is a he or she. I actually thought my own male cat was a female up until he was six months old. Like I said, be sure to take him to your local vet to eliminate any misinterpretations of what the real problem may be.