Bulldog losing hair.

hello,
I have a 3 year old english bulldog that has had skin issues since he was about 1.5 years old. He loses hair in circular patches all over his body with flaky dark skin. These are pretty common on his sides at the shoulders, ribs, and rear. He has had an airborne allergy test at 2 years old, which they found nothing to indicate allergy shots. He is also currently on Z/D ultra allergen free diet. Yet these spots still appear on him regularly without any seasonal connection that I can tell. He has chronic ear infections and seems to be constantly itchy on these soars. Just recently he has had some scabs and hair loss on the edges of his ears. In the past the vet has scrapped (many times) for mites on these spots with no indication them. As I'm trying to eliminate possible causes I am left with: Mange/Scabies, skin infection, Endocrine Disease. He is going back to the vet today, but I would like to inform myself as much as possible on the test they need to perform and what the accuracy is of such test. Any help would be greatly appreciated.