If all medical and parasite issues have been ruled out then try this brand or something similar.
The Honest Kitchen dehydrated raw food. You may want to call them directly and ask which kind they will recommend, they have several varieties with different proteins.
You can also try a home cooked diet for a few days, see if there is any change. Like boiled chicken and/or ground beef with an equal portion of soft cooked white rice and a little broth. You may also want to ask you vet for some low residue canned food. It is very expensive, so you'll just want to do it for a short time to see if an easy to digest food will help. It may just be the kibble (texture) is irritating.