My 9 year old male cat has lost 4 pounds in 8 months. He is constantly hunger, eats 3 cans of wet food and free feeds Hills ID. Vet tells me he has FIP but when I look that up hunger is not a symptom, lack of appetite is a symptom, he stools lot as much or more than he eats.

How did the vet diagnose FIP?
Did his thyroid get checked?

What other symptoms did you or the vet notice? How long has this been diagnosed? Why was he eating the Hills ID food?

Sorry kitty is not feeling well. Hard for you too... He need to be evaluated for diabetes and many other things. It would good to take him to the vet, now.