One of my cats got very skinny very fast, but it was from a pancreatic disease that came on very fast. When he got down to 3 pounds, unfortunately, I had to have him euthanized becuase he couldn't eat. He was 18 at the time. I hope you feline improves and that they are able to come up with some answers for you. I hope your cat enjoys a long life and that whatever is currently ailing him will be rectified soon. It's almost as if your cat doesn't have a proper digestive system., or his food isn't going into the digestive state like it should. Keep us informed.