Growing a horse's mane out?

I own an 8 year old bay quarter horse gelding out of Rowdy Yankee. He is my pride and joy in reining I just wish he had a longer mane like my blue roan quarter horse mare. He only has about a 5 1/2-6 inch long mane, while my mare has a mane that's a foot long. I wish he had her mane. I have tried MTG and I really can't stand the greasiness and the smell. I have used eqyss megatek rebuilder and it did help his mane grow a tiny bit but it took forever! I want to be able to add a good couple inches by mid June. 10 points to best answer.