PotentialGSDowner...kudo's to you for recognizing a despairaging situation for this poor dog and perhaps taking on the responsibility to give him a new lease on life.
I think with the right protocol (radical to some, normal to others), this issue can be turned around.
Keep in mind that allergies are a symptom of the immune system. The best way to boost and assist the dog to naturally recover would be to feed RAW with an antioxidant boost of some pureed green leafy veg...some digestive enzymes, probiotics raw organic coconut oil and fish oil.
Carmen - who posted, has a line that will cover noted along with added herbals that detox the kidney and liver as well as reduce inflammatory conditions and support and compliment a raw diet - and I suppose, a good quality kibble (secondary), but focus needs to be on bringing the dog back into balance - the gut, bacteria and yeast are prevelant and sugars have to be removed - that is in the form of carbs...there is NO kibble that is carb free.
The choice is ultimately up to you if you can or want to take on that resposibility - but it WILL do the dog better and for all - his health and your wallet in the long run.
The people must have had him in and out of the vets a ton - so it likely the dog has been over-vaccinated, this is a contributing factor in immune response to foreign invaders syndrome - "allergies".
An herbal detox, raw food, good suppl. exercise in the sunshine and a caring compasionate family...this dog will be happier then a pig in poop.