According to our vet, our cat is allergic to something. He can't eat without howling because of pain from chewing. He's getting thinner and thinner. The vet guessed he was alergic to fish, but it's no longer in his diet, and he's still got the problem.
Dental problem? Impacted gum disease, or rotten tooth?.......
Have you had his teeth checked? I had a cat with bad teeth and some had to be removed. They can't clean their own teeth and they can't tell you when they've gone bad.