but I assume the vet also ruled out other skin conditions (mange, ringworm, insects, etc.). There are other diets the dog can try (raw diet, etc.). I wouldn't ordinarily think of Purina as the highest end hypoallergenic food. Also, make damn sure whatever he's eating isn't made in China.
Do you bathe the dog? If so, what sort of shampoo do you use? Make sure it is also hypoallergenic.
Also, if the dog is outside a lot, make every sure to keep him off of any grass that has been treated with pesticides. You may want to check your house as well. Chemicals on the rugs, floors, etc., can really bother a dog. Even the stuff used to clean their bedding.