The symptoms you describe such as excessive drooling would have me concerned enough to go to the 24/7 nearest emergency veterinary clinic, rather than wait 3 days for an appointment. He may need medication asap to stop the suffering, he sounds miserable.
Some of the other symptoms sound like environmental allergies (not related to food)
Have you checked the search engine here?
Btw: I would not apply anything to the dogs skin unless a veterinarian that has examined the dog advises you to do so.
Example: What you describe sounds like environmental allergies, food would have little impact, if any, on this condition.
I would continue to work with your veterinarian, however, for best results, I would go to a specialist, a veterinary dermatologist.
Have you tried the search engine here? This subject comes up frequently.
Example: /forums/topic/irritated-skin-food-allergy/
/forums/topic/dog-chewing-nails-till-they-bleed/
“Food allergies are rare. Food sensitivities tend to result in GI disturbances such as vomiting and diarrhea. Environmental allergies tend to show up as pruritus, ear infections and such”.
