B-12 is a water soluble vitamin excreted through the kidneys if in excess.
Which form are you giving him and why?
If it is cyanocobalamin to control or prevent pancreatitis, I'd give it, as that could knock him out of remission if it flares.
If it is methylcobalamin to remit diabetic neuropathy and he is not displaying any symptoms, you probably can omit it.