My dog itches way to much.Waaay to much.?

He's a 4 year old German Shepherd. Like I said he itches way to much. There are two black(with a tiny bit of red) strips on either side of his back where there's no hair. He bites himself, scratches with his nails, and rubs himself against objects. His back, under the ears, and paws are problem areas. Where ever he itches, there is black and no hair. He often breaks his skin and gets blood out. Recently he damaged the pad of his front paw so badly that he can't walk. We use a hypoallergenic shampoo. We've changed his food four times. It went from beef to chicken to vegetarian, and now it's on duck. We don't spray him with anything and we've already taken him to the vet many times over because of this. Our vet doesn't have a clue. We're in the midst of changing vets amd I was wondering if anybody out there maybe knows something. Please no immature responses because this is a serious problem. It's heartbreaking. Thank you.