Oh! Love this question! I say invest in a tail bag, it helps the horse to stop damaging his own tail. Then get him on some supplements. Healthy horses= healthy mane and tail! Good luck!
Unfortunately I'm having the same problem. my trainer told me that we are going to try to use a weighted tail in the show season. Weighted so he wont hold his tail so weird and a fake tail so it's nice and full. So you could always try a weighted tail :) hope this kinda helped you