My boy rubs his tail during the heaviest part of fly season. It's kind of a vicious circle because rubbing breaks off hairs and now those short little hairs poke the inside of the butt cheeks causing them to rub even more.
This year I bought one of those yellow sulfur and olive oil bars; lathered up the tail top side and bottom and left the lather on for about 15 minutes. After rinsing I was happy to feel the olive oil left behind to moisturize. The sulfur really helped with the itchiness and the rubbing stopped.
Now I just use an ethnic hair care spray that has argon oil, tea tree oil and other goodies. It's been a few months and the tiny broken hairs are now grown out enough to begin to hang down and no longer poke his bum.