I live with four indoor cats from the same mother (two females 9 years, two males 10 years).
One of them has a problem with the urinary tract since birth and has therefore had to eat Hills C / D chicken flavor and since they have free access to food, everyone has eaten it.
Now it is the case that the other male got a rash on his back and when we went to the vet he said almost immediately that it was due to a food allergy, so the cat got a shot against the rash and the advice to start eating according to the Hypoallergenic diet.
The question then is whether the cat will need to eat according to the Hypoallergenic diet for the rest of its life, or if after a while you can possibly try to give him some regular cat food (not C / D)?
If the cat really has a food allergy (allergy to any protein in the food), he will probably not be able to eat c / d again.
However, you can not diagnose food allergy by just looking at the cat, but you must do an allergy investigation.
If your veterinarian does not know how to do this, you should contact another veterinarian who is good at skin diseases as well.