Your cat could have a rash, or some sort of allergy to something in the house, the food he eats, or anything else that might cause trouble with his skin. It could also be a mite or fleas that will do that. Another one is ring worm that could start small and grow. Go to the vet.
Hair loss has many causes, all of which need to be determined by a vet. It could be as simple to treat as an allergy or a parasite, or it could be as serious as Cushing's or Addison's Disease, or a thyroid problem. Lab tests will tell you.