Joe Joe rubs his mane and tail also, and I agree that the MTG isn’t as helpful as I had hoped (besides, it makes him stink and he hates it). The only suggestion I have is to make sure you keep them as clean as possible, to eliminate itching. Others I know use a sleazy thing to prevent mane rubbing, but he hates that also, and rubs even more. He only does it during the summer though, so perhaps since your boy does it in the winter a hooded blanket might help with the mane? I use one with a detachable hood, so if he destroys it I can still use the blanket.
It is never the horse's fault