Play with his mane, alot. Use diluted conditioners, and finger rake, then braid his mane. This will stimulate his mane to grow. I've just gotten 2 new geldings, and their manes and tails were chewed off by a pasture mate. I've done the follow, and my paints mane/tail has grown 2 inchs in 4 weeks.
Each day I soak down his mane and tail with water. Followed by spritzing diluted Sauve human condition and Mane and Tail conditioner, soaking it, really. I use both very diluted.
Then I alternate what I do now. Either I finger brush out and braid into a style that excites me at the time, or I brush our with a wide tooth plastic comb, very very softly. Brushing to hard will break the hair.
The next day, I pull the braids out, and rinse and repeat. I also will spritz a mixture of vinegar, Listerine and water onto the hair prior to rinsing.
Once a week I wash him with a soft mild soap.
~*~Never approach a bull from the front, a horse from the rear or a fool from any direction. - Cowboy saying~*~