There has to be a reason that he is Tail Biting. Are the Perimeters in Safe Mode? Is is Tank to small for him? Are there enough Hiding Places for him?
To find the Cure, you must find the Problem first. Anyways, Betta Fish are either Scared or Stressed out for Tail Biting. Make sure he feels secure. And since he made an open wound, you have to prevent Infection. You would have to make the Water VERY clean. And make The Temperature 78F-80F.