I'm sorry to hear that he's not doing well. I had a cat that would have severe diarrhea and he also became very thin because everything he ate would come out of him again. This was my RB Starr and he was diagnosed with IBD,irritable bowel disease, and also colitis. He was put on prednisolone which did help but later on he gained so much weight that he could barely move. Yes, I would definitely go back to your vet and have some blood work done or at least talk to your vet more about Gabby. Also ask your vet about vitamin B12 injections. These really helped my RB Starr and they also help to increase the appetite and help to increase energy. Good luck. I'll continue to keep Gabby in my thoughts and prayers.