I don't completely agree, sorryB-12 injections work IF your kitty is inf fact b-12 deficient, which is the case with some IBD kitties, but not all - you can test for b-12 deficiency easily - please do that before doing a trial with the injections..... There is no need for them if he is not deficient.
http://www.felinenutrition.net/b12.html
Cats with IBD or a history of IBD are at risk for Vitamin B12 deficiency. Vitamin B12 is absorbed very slowly in the intestines via a carrier protein called intrinsic factor. The chronic inflammation caused by IBD interferes with or may completely block absorption of Vitamin B12. Since daily requirements for B12 are low, the body's reserves can last for years even after absorption from the diet slows or stops, but eventually the reserves will be depleted and the cat will develop symptoms of B12 deficiency. Testing for serum B12 levels may not reveal cats at risk for B12 deficiency symptoms, because the levels in spinal fluid and at the cellular level may be much lower than serum levels. According to the Gastrointestinal Laboratory at Texas A&M University , cats with IBD and other gastrointestinal diseases are at high risk for tissue-level B12 deficiency even if serum levels are normal.