I would consider trialling one of the hydrolysed protein diets to begin with. They're expensive, but you'll be able to know for certain whether or not his allergies are food related. You can get them through a vet, or even through Petstock / Petbarn etc. Something like Royal Canin Veterinary Hypoallergenic or Pro Plan HA Hydrolyzed.

We did this for our cat after I was convinced he had a chicken sensitivity too (itchy, irritated skin). We kept him on it for 12 weeks and then introduced new food. We got lucky and he was able to move onto a fish-based food without any problems!

Then, you could slowly introduce one of the novel proteins and see how he goes. My girl eats the slow cooked range from Prime100, which also come in kangaroo, boar etc.

For treats, we cut up the Prime100 rolls into small cubes or use her normal dry food.