A friend of mine whose White Boxer suffers allergies switched her dog to California Natural dog food with great success. Her Boxer is allergic to grass, mold, most foods...I some times thing the dog is even allergic to air!
California Natural is a very basic food and is good for a dog with allergies who can't handle a big amount of ingredients. Since the body doesn't have so much to deal with this will allow you to strengthen the immune system.
Another good food to try is EVO (you can rotate, first feed California Naturals for a simple food), EVO has no grains, grains in dog foods or too many carb's can cause an over reaction to the immune system. With one of my Newf's which suffered from Aural lick dermatitis, I took her off all grains and Carbs and she did wonderfully after that and her paws healed up nicely.
I would also try to build up the dogs immune system by giving immune enhancing mushrooms like Shittake, Maitake and Reiska. Discuss these mushrooms with a holistic vet first to make sure your dog doesn't take a reaction. I myself add these mushrooms into my dogs raw food diet with great success. Garlic cloves is also good as an anti bacterial and anti fungal and builds the immune system. Give in moderation.
You might want to look into adding Astragalus to the dogs diet to help build up the immune system. I never advise a person or dog with allergies to take Echinacea as this can over stimulate the allergy response.
Try to add more omega 3 fatty acids into the dogs diet as well, including sardines, salmon, flax oil/crushed seed, and hemp oil. I would also try giving more raw foods with natural digestive enzymes instead of only feeding killed and over processed kibbles and canned dog foods all the time. Only feed organic foods as the body won't have to deal with all the toxins with mass produced livestock and produce.
If I were you I would also order a few holistic health care books to help you with your dogs diet.
Here is a page where you can order books
http://www.sitstay.com/store/books/diet1.shtml
The books I enjoy and own are: All you ever wanted to know about herbs for pets, The complete Holistic book, Healing Pets with Natures miracle cures.
I really love the Natures miracle book, there is some fantastic ideas in this book for allergies, Cancer etc.
Good luck