Saline, is not table salt. Saline does not kill good skin, Saline, is a cleaning agent used by Vets, and Human Medical Staff, like me.
We are not Vets and cannot diagnose.
You asked food allergy, if that is the case more of his body would be affected.
How the at acquired the wound is not of importance, (unless he is not up to date on Rabies shots)mandatory in most states for Cats, since they are one of the
main sources of carrying and spreading Rabies), that being, said, identifying is also not important. What is important is treating the wound.
Cat needs to see a VET.