Aural Hematoma on one ear of my Maine Coon

[Catster]

Took my boy to the vet with a Aural Hematoma..Vet checked his ears all clear but was a little red..started him on Octic HC 10% for a week but it only seems to be getting worse..it helps with him not scratching his ears. But he is still scratching his neck...NO FLEAS.. I think he has an allergy to something being given via food.. He is 14 months old was on Royal Canin baby and mama .. Then switched him to Science Diet altho he just started the Science Diet I think it was the previous food..Was also given wet food.. Any Suggestions on what the most common allergy in the food for Maine Coons is please ?? I am not feeding him seafood as I was told this was not good for them when I got him. Any info is appreciated.. Thank you !