Hmmm, my only thoughts are worms or a dirty/irritated sheath. When my guy would rub his tail in May, he'd get a power pack of panacur. When he started rubbing it at shows, it was due to the shavings irritating his sheath. Once I figured that out, I started bedding his stall down in straw at the shows. Now that he's retired, if he starts rubbing it generally means his sheath needs to be cleaned.
The odd time the skin on his tail will be gunky so I'll wash his tail with a good shampoo, rinse super well with cool water, and then pour a rinse over it - either regular listerine, apple cider vinegar, or spray it with a homemade concoction of conditioner/baby oil, etc. Some people ust MTG but my guy is too sensitive for that.
Consider having him do a skin scraping, too, even if it looks OK. The other thing I'd recommend checking is thyroid function; problems in that area can cause various forms of hair loss.
Also think about whether you notice a seasonal worsening of the problem. One of my mares had dreadful tail loss issues for several years, tending to worsen in fall/winter. Vet finally found it was linked to a thyroid malfunction that was most noticeable at that time of year, coupled with an annoying folliculitis that took advantage of the conditions created. With both problems treated, she now has a normal tail, if not a glorious one (but that's just not in her genes!)