I am sorry that you have been waiting for a response. I recently just logged onto the site and noted that your question hasn't yet been answered. I am a licensed veterinarian with over 25 years’ experience and would be happy to work with you if you are still needing assistance.
There are a couple of things that may help. Most cats with GI disease do not absorb Vitamin B12 very well from their gut and have very low circulating levels in the bloodstream. Vitamin B12 (cobalamin) given by injections does help with weight and appetite. We usually teach the owners how to administer at home. 250 mcg given once a week for 6 weeks then every other week for 6 weeks then once a month.
The medication Cerenia, developed as an anti-vomiting med, given at a higher dose of 2 mg/kg is very effective as reducing inflammation in the gut and will help the prednisolone to keep things under control for a longer period.
Other supplement additions can be of benefit especially those that contain zinc. The product called NuCat Senior is a nice one in that it does contain zinc, taurine and B vitamins. LINK HERE
I would still supplement the B12 with injections however as the amount in the oral supplement is very small and as mentioned, these cats do not absorb well from the gut.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.