You cat is still young. Find a veterinary surgeon
in your area who can place a
peg tube directly into your cats stomach, so that he can be tube fed. Have that vet take blood to do a test for Bartonellosis. Request that your vet place your cat on Zithromax. If you are not interested in having a peg tub placed than you must have your cat force fed and placed on IV fluids, at least, at an emergency hospital.