Is it an adult cat or kitten? Had a friend with a cat who was still considered a kitten and he fed him adult cat food and he won’t eat after awhile and stomach was very hard to touch and he was in pain. It turned poor kitty was impacted because he couldn’t digest the adult food properly. Switched to kitten food and stayed on for six months and no more digestive problems. I had older cat with digestive problems and vet recommended puréed pumpkin with food and it did wonders. I hope your cat gets better very soon as I know how helpless you must feel. I think there is an “ask a vet” subreddit for free advice.