I've got a tail biter too. From what I've read about it on this forum, bettas who bite their tails either are reacting to something or they're just fed up with their long fins. Try everything you can before throwing your hands up and saying "he's just a biter" though he might just be one of those. This is some of what I've tried for Falkor, my tail-biter:
* Low light
* Lights off
* More decor/plants
* Less decor/plants
* Moving him to a higher traffic area
* Having him in a quieter area
* Moving decor around during water changes
* Not moving anything in case it stresses him
* Bigger tank
* More attention
* Covering his tank at night
* Covering all but the front of his tank
* Changing what is surrounding his tank
* Less attention
* Different food
Pretty much the only thing I haven't tried is housing him next to my other fish, but I'll have to try that when I'm about to attempt cycling my tanks, because I don't want him in a cycling tank while his fins are gnawed off.
After trying all of the above, I've basically resigned myself to making sure he's got nice, clean water, IAL in with him, using Stress Coat for every water change and basically letting him clip his tail to where he wants it, because seemingly nothing I can do will stop it.
Try everything you can for your tail-biter, and when you've exhausted all options, just make sure his water is always nice and clean, and perhaps get some IAL to put in his water.