Have you checked with your vet to see about a skin condition called mange? That can cause their hair to fall out. Is he licking himself and causing the hair to fall out? I used to have a cat who continually licked himself until he bled. I bought a product called Fooey which is really bitter tasting. I sprayed it on the areas that were the worst and he quit. You may want to find out if he's doing this to himself or if it's some type of skin condition. Good Luck!