Definitely for long term I would use some kind of conditioner/mane grower...I personally don't like MTG, but if you don't mind it, go for that. Otherwise just find a good coat conditioner, and use that weekly. I also like the idea of something 'internal' as suggested above...skin and coat is only effected 'short term' by conditioners, and other external products, but if you can feed him something that is specific for hair and coat, you may get even better results.
I think the yarn idea is a fabulous idea; I think that could definitely work to keep his mane together in the braid...all you can do is try it and see!
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."