itchy dogs

I have four small rescue dogs.  All tend to lick their paws a lot and the youngest is constantly scratching his whole body and licking and chewing his underside and legs.
He has been to vet several times, we have put fish oil in his food, we have sprayed with anti-itch spray and have also tried a prescription for allergies from vet but still
no relief for him.  What are your thoughts on food allergy and what brands seem to
alleviate these symptons??