I'm so sorry to hear this. My RB cat Starr had colitis and IBD so I can relate to what you're going through. The only thing that helped him was for him to be on prednisolone but this made him gain so much weight that it made his life miserable and it shortened his life. I hope that you'll be able to find the right solution for his problem. Maybe a holistic vet might be able to help you. Prayers and positive thoughts are being sent your way.