Don't brush your horses tail! Leaving their tail alone and not brushing it will prevent ripping out hair, which you want to keep! Try using MTG and a tail bag. If you use the MTG every few days and then, when you bathe your horse condition their tail and leave some in there to help the hair stay smooth and grow.
Don't brush it unless you have to, mtg worked on my girl and I did it just 4 times a month then conditioned it the other days. Also there are supplements you can give your horse to help the growth (Smartpak has a couple).
We feed Animal Element Detox, which is a wonderful product for any horse and it really promotes hair and hoof growth!!The horses bodies just get really healthy and they begin to be able to utilize their nutrition properly and their mane, tail, and hooves just grow out so nicely! Also, you can use MTG on a regular basis, really massage into the horses tail bone area to make sure it really absorbs in. It smells quite bad, like bacon Grease really. But, it works to make the hair grown faster and stronger.
Brush it out with your fingers and lots of detangler so so it doesn't catch and get pulled out. Also you can moisturize the doc. To kill the bacteria on the doc that may make it itch or hinder hair growth, you can massage in listerine (mouthwash). If you do that then be sure to check the tail doesn't get irritated or too dry, this can be done by moisturizing.
These things will just help it grow healthier and longer.
Sadly there isn't any magic formula to make tails grow faster.
Bag it and/or tie it up and don't brush it. If shavings or something gets in it just use your fingers to get it out! He may be rubbing it out when your not around so if it's itchy you can get some MTG and drench it with that it will help it grow faster