My Dog Is A German Shepherd Lab Mix With Terrible Skin Allergies. What Is The Right Food For Him?

Allergies are about control not cure.  The three types of allergies are food, flea, and environmental--often dogs will experience a combination of these allergies.  You and your veterinarian need to form an allergy treatment plan together.  Allergies treatment is multi-modal and should consist of some of the following depending upon your dog's needs: Anti-histamines, omega-3 fatty acids, frequent bathing with a medicated shampoo, steroids, allergy injections, immune modulating medications, allergy diet, and medications to treat secondary infections (bacterial, fungal, or mites). 
There are 2 types of allergy diets--novel protein and hydrolyzed protein.  Novel protein diets are made up of ingredients not normally found in pet foods--this means your dog has not been exposed to them; for example duck and potato or rabbit and green pea.  Hydrolyzed protein diets have the protein source broken down to very small parts that should not trigger an immune system response and hence an allergic reaction.  These diets are available at your veterinarian's office.

Are you sure that his skin problems are due to a food allergy???  You can try any food recommended by your vet, but your dog must be on it for at least 30 days to give the food a chance to work.   Some vets recommend a fish and potato diet, because most dogs are not exposed to fish and potatoes in their previous diets.  Check to make sure your dog does not have fleas and if the new diet does not work, there is allergy testing that can be done from a blood sample and that will tell you what the dog is allergic to and therefore you can find out if it is even a food allergy.

I don't know much about the food thing, but I would try giving the dog vinegar water baths it is soothing to the skin. It isn't harsh like soaps and you can use it as often as you like... The dog will only smell like a salad until he dries and then he will smell fresh and clean.

What type of allergenic shampoo's r there? I am trying a sweet potatoes food...