A picture would definitely help if you can get your camera charged. But even if his is biting his tail you should still treat him, when they bite their tail the edges of the potential to become infected, causing finrot, so either way treating for fin rot is a good idea. I've found the best treatment for fin rot is aquarium salt, clean warm water, vitachem (I swear by this stuff, best for fin rot tratment and fin regrowth!) and a high protein diet! If you can soak his food in garlic juice (either garlic guard or garlic juice with no preservatives) before feeding it to him that will help give his immune system a boost.
Bettas that tail bite usually do it out of boredom or stress. What is his tank like? Are there plenty of decorations for him to occupy himself with? What is the tank size/temp/ammonia nitrite and nitrate readings?