only about 2% of dog allergies are food related, rest are envriomental. Only way to know for sure is to take him to a dermatologist for allergy testing.
Signs of allergies are ear infections and swelling of the ear canals, chewing the feet and talk area.
I don't envy you, trying to find the cause of the allergy can run into a lot of money then allergy shots to keep itching under control are also expensive.
If its the grain in the wood try switching to a non grain, or raw food. If that helps then you know its food. If not then its something else he is allergic to, could be anything or a combination of things.
Good luck, I once heard of a horse that was allergic to hay. imagine dealing with that?