Cat very tender in lower stomach/back/side

[Nicolevh]

Our 2 year old male cat has all of a sudden gotten very sensitive to touch in his lower back/stomach area. So much to the point that he growls deeply, hisses, and will meow with his mouth open wide, all the while looking at us. Then he'll retreat beneath a bed or table and come out. He still will jump up on our laps and will want to be stroke around his chin and ears, but if we ever touch him elsewhere he goes crazy. What can it be? It has been like this for 3 days. The kids think he is losing fur and he does appear to be thinner, but he eats whenever he comes in the house.
Thank-you