Hi, and so sorry that you are having this problem and poor kitty.
did the doctor mention anything about granuloma, or rodent pumps? it is a form of allergy, or sometimes a parasite, and it is treated with prednasalone, primarily.
your cat may benefit from a grain free food. there are some good ones at pet stores, like wellness grain free, with duck. some cats get relief when they go off grains.
you might get him to a specialist, for it may be cheaper in the long run. often, your everyday vets are not specialized in things like this, and you may spend tons on things that are just guesses on the vets part. and your kitty will feel bad in the mean time, and you will be too.
my best wishes for you and kitty,
chicory