My dog's had same symptoms for a few years now. Multiple vets have been pretty useless. Some say diet....some say it's environmental.....one even said it was the water he was drinking. We've changed diets, tried raw food diet and every other food from cheap to uber-expensive, given rain and bottled water to drink and bath in, moved to a different state, rubbed him down with oils, bag balm, bathed him in Epsom salts....and he's still the same. If we leave him alone, he'll scratch himself bloody.
Vets gave him a steroid creme and benadryl, but it didn't help much. So far the best thing has been the Epsom salts. It keeps him itch free for a couple days, but he hates the tub so there's only so often we can bath him.
I feel bad for him. He's gettin up in years (11 or 12) and he's slowly getting worse. He's miserable most of the time. We've thrown more money at it than I care to think about, but don't know how long that's gonna last.
May have to take him for the long walk one of these days.