How to tell my vet that I think he's wrong?

We have several IBD cats in our clinic that actually improved drastically after removing all dry food from their diets and switching over to a grainless raw one. If you are ok with medicating I would definitaly talk to your doctor about switching back over to a raw diet since your cat had been doing well on one before and is showing signs that a 'normal' diet is just not for him. If you do decide to switch back over to a raw diet and medicate you will end up with a corticosteroid to relieve the bowel inflamation, most likely prednisone, and an anti emitic to contrtol vomiting.