The best way I know to grow a mane is to treat it very gently. Don't ever take a brush or a comb to it, this pulls the hairs out and breaks them. When the mane is tangled, spray it with some detangler and take the hairs with your fingers and carefully seperate them. Don't use your fingers as a comb but think of it more like how you would manipulate the while when your braiding sort of, if you know what I'm saying??? Just be very gentle with it and keep it shampood and conditioned. I promise this will give your horse a beautiful mane.
MTG is a sulfur-based hair growth formula. MTG is the official name, and stands for Mane Tail Grow. Google it, and you can find many places to buy it. If you have a favorite tack store, they might have it, too.
Good luck, and I hope we get to see some before and after pictures!