Our first dog had a few allergies along with his thyroid problem. Some of his bare patches never grew hair. He had the hardest time with allergies in spring and summer. Grass seemed to be the worst for him, and of course he loved laying on the grass.
He would get other hot spots throughout the year and we would treat those individually, keeping them clean and sometimes using topical anti itch powder like gold bond.
Sounds to me like you are bathing the dog far too much which will only make his skin worse. We never bathe our dogs, but for anyone who does, more than once a month and you're asking for skin trouble.
Feeding a fish diet can also make their skin bad.
clm