Hi there. I had a tail-biter. You really have to do some detective work to figure out what could be causing him to do this. It might mean watching him for a while when he doesn't know you're there, or finding a way to monitor him so he can't see you (check out his behavior when he's alone). You really also need to keep his water verrry clean. Clean water at the right temp will go a long way toward helping him heal. If he freaks out when he sees you, try blocking his view for a while. If he paces the tank, try breaking up his path or distracting him with lots to do & explore. If he has a light, try dimming it.
For my little guy (Uncle Sam), it was really obvious what was causing his tail-biting... stress. I had him in a 10 gal divided tank and no matter how much I tried to block their view of each other, they became so stressed out I had to completely separate them. I make sure they have lots of distractions, interactions with me and things to explore. He stopped biting as soon as he had his own place and his tail is slowly growing back.