My suggestion would be to look into his skin condition. There is a plethora of things that could be causing this. It could be that flies and pests are bothering him, some sort of crud or bacteria, an allergic reaction, dry skin... etc etc. Does he show any signs of rubbing out the top of his tail? Does he ever have reactions to bug bites (swelling, obvious discomfort)?
If he's rubbed his mane completely off, I think it would have to be more intentional than just brushing against the fence trying to get grass. He may be itchy or uncomfortable in some way. My suggestion is to start of with getting his whole mane as clean as you can. Give him a bath and really work the shampoo into his mane for a few minutes, then rinse it off and keep rinsing until you're certain that there can't be any soap left.
Cowboy Magic sells a product called Krudbuster that may or may not help depending on the cause of his rubbing. I used it recently on my mare's tail because she was rubbing it out at the top and she does not appear to have rubbed it since. If it's just that he has dry itchy skin, you can supplement his feed with a cup of black oil sunflower seeds (the type that are sold for bird feeders) every day. The sunflower seeds usually improve skin and coat condition. You can read more about them here: Black Oil Sunflower Seeds for Horses
Finally, make sure he's thoroughly protected from pests with plenty of fly spray.
The MTG will probably help with growing back, but you're right... you need to resolve the cause before you can hope to recover from it. You can take a thorough approach and put all of my suggestions into effect immediately. Nothing will hurt him even if it isn't entirely necessary, and none of it is very costly. If he starts to improve then you're in luck! If he does not improve or worsens, call your vet and get a professional opinion on what is causing the problem.