Is he still rubbing or are you just wanting the hair to grow back from when he rubbed it off?
If he's still rubbing and has been wormed and had his sheath cleaned, I would spray some Banixx on there once or twice a day and rub it in. My gelding rubs his tail for apparently no reason. He has been tested for worms and has never needed to have his sheath clean in the 3 years I've owned him. I have my vet check it every time he gets his teeth floated. Anyway, I tried MTG on him and found that although it was probably helping the hair grow, it didn't stop the itching so he kept rubbing and ruined all the new growth. The Banixx helped wonderfully.
If he's not still actively rubbing, I would use the MTG. It works great, but make sure you hold it upside down and shake it until all the sediment in the bottom is mixed in. It won't work without it.
Good luck : ]