I know Hills has a prescription diet you can get from your vet for IBD.
You can either get it in kibble form or canned.  You can soak it in water
and add the turkey or rabbit to it and put in blender to make a nice soup
or gravy, depending on how they prefer it (thick or on the watery side).
I did this for one of my ferrets that had IBD and he was also on a low
dose of prednisone also.  Hope this helps.

My sincere condolences to those who have lost a ferret.  My heart goes
out to you.

Eleanor, Donald and the 6 pack of ferrets
[Posted in FML issue 5281]