To some extent, a chiropractor can help with back alignment which effects tail set. I don't really get the mechanics of it so I'll show what happened with my horse who started out carrying his tail to the right of him to counterbalance his slightly uneven hind.
A week after I bought him, notice that his dorsal line and the high point of his hind don't match up, as well as the dorsal line being more on the right side of the tail:
Two and a half months later with good riding, muscle development, 2 chiro visits, and 2 masseuse visits. The dorsal line and the high point match up, the dorsal line goes evenly down his tail, and he no longer holds his tail to one side when he's moving around, although you can still somewhat tell that is used to be held to the right (will improve with time).
From my own experiences, chiropractic work did help with his tail set, because his hindquarters were corrected. I would say go ahead and ask a chiropractor to come out and take a look at the stallion, it certainly won't hurt him.