First see if there is a pest bugging him, ticks, lice, worms. Etc.treat the cause. Wash the tail with an antidandruf, Then use milking cream, coconut oil or a specialized hair growth product (avoid Vaseline, I've heard it can block the pores and make the hair fall out..) good luck ☺️
Before putting products on, I would try to figure out the cause first. This is really commonly a symptom of a sheath that needs cleaning, so I would definitely give him a very thorough clean and be sure to rinse well. Hopefully that'll solve the issue!
Another thing it can indicate are worms, so be sure to look into your working schedule and fecal counts to see if he might be in need of a deworming