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

Just take the puss cat to the vet and be very honest. Explain that the vomiting and constipation has been cured but that you are concerned that your cat is not interested in his food and he has a minimal amount of energy. Ask whether you could gradually introduce other food items such as chicken into his diet and if you did would his IBD return. I should add that I know nothing about IBD in cats but this is the way I would ask the question. Good luck.