Can dog food cause skin allergies?

My dog is eight years old. Over the last 6 months he seems to be constantly itching. I know that it is not fleas ticks etc. He is on Advantix and the vet who was convinced it was fleas couldnt find one and cant seem to help me with this problem. His answers is simply keep giving him benedryl.... Someone told me it could be the food I am feeding him. I switched him to Purina One Healthy weight formula about 8 months ago from Benefull. Could this be the problem and can anyone suggest a good reasonably priced food to switch to.