My boss was just telling me about a clients golden retriever. Apparently he was the same as your dog ..he had terrible allergies and had exhausted every therapy. As a last ditch effort, they slowly switched him to the BARF diet (raw food) and the dog got 80% better. Can't say it would work for every dog but it's something to think about.
Caretaker of 2 dobermans,, 1 schnauzer, 5 cats, 3 goats, 1 parrot, 1 bearded dragon lizard, 11 chinchillas.
Rest in peace, Cricket, Casey, Bella, Yank, Chloe and Bones.