Many people of this forum use a product called MTG. I've heard it is amazing for growing new manes and tails.
I have been loosely french braiding down my horse's tailbones down to the end of the tail (leaving a switch for flies). It helps to keep the hair from breaking off from whisking flies away--not sure how well it would help with growing new hair though. With a braided tail you don't have to brush it out very often (every week or two), so you can minimize breakage.
Make sure you stand as close to him as you can when working with his tail, especially if he doesn't like it. It is much better to get shoved with their hock than to receive a full blown kick!
I got lucky with my Appaloosa in the hair and hoof department--his mane is past the bottom of his neck, and his tail is full to below his fetlocks. It must all be in his genes!
| Kubie, Appaloosa (RIP) | Patches, Pinto Arabian Pony | Scotch, Paint Quarterhorse |