Do you know what he's allergic to?
Chicken has been implicated in a lot of dog food allergies/sensitivities. You might start with just going to a food with another protein source and see what happens.
But the problem could be with the rice or any other component of that food....unless you know, you're going to be doing a trial & error thing. Also, if it's chicken, you have to watch for chicken ingredients even if the primary protein source isn't chicken.
Good luck!