It really depends on what they're allergic to. If the chicken and rice doesn't work they we try lamb, if that doesn't work they have a whole line of odd food for him to try, one was duck and potato. sheesh. He wasn't bad before the switch, so hopefully once he heals he'll stay good, unless he's just developed a weird new sensitivity to something. We're sure tired of it already!
When Gifford had his alergies it was kinda the dark ages of foodferdogs. We had to get the mutton/rice from the vets by the case. Not that it mattered, he needed it but there wasn't much else available then.
Good Luck on finding the right combination for Homer. Our furry companions can be persnicketity.