If your regular vet doesn't seem to be able to help you you can always try asking another vet about what to do. I have my favorite regular vet, but when it comes to some problems I'm often not happy with her diagnosis or treatment. Ive taken my cats to other vets then to get the help my furry friends need.

How long did you give your cat to get use to the food? It may take a while. I would try for a good 4 days and watch and see if he is eating a bit more every day. Sometimes they warm up to a food once they know its all their getting.

Also, Ive had cats turn down Science Diet, but when I tried a brand called Selective Care they ate it right up. Your vet could help you get the right formula from Selective Care since they make many for many different health problems.
Please keep us informed and good luck!