What are the ingredients of his current food? Nutro Natural, Natural Balance, Natural Organics, too many with Natural in the name. Corn, wheat, and soy are known allergen producers, but it can be the protein source like chicken and you have to switch to turkey or lamb.
Adding Salmon oil or another omega fatty acid supplement helps with skin issues.
I would also make sure he is worm free as that can cause skin problems.
You could also talk to your vet about giving him Benadryl to see if that helps. Here is a link discussing its use in dogs.
Dog Allergies Treatment with Benadryl