He could be stressed, bored, or your filter (if you have one) is making it difficult for him to swim. That was what caused one of my male bettas to start biting. He seemed fine with the filter for a couple of months but then it got gunked up and the current got stronger because it was coming out of the overflow area. I treated him in a 1 gallon tank and fixed my filter, and then he was fine for a couple of weeks after I put him back in the filtered tank, but then he started biting again.
Sometime soon I want to get a divided 10 or 15 gallon for my current three bettas, and I will put him on the opposite end from the filter where there isn't much water movement.
Otherwise, if you don't have a filter, he could be bored... try adding more plants and decorations, or moving your current ones around- give him a change of scenery. Also put a ping pong ball in his tank. I hear they like playing with those.
Otherwise, if it's from stress, I'm not sure what you could do. Does he have any tank mates that might stress him out? What is his water temp? Is he ever in direct sunlight or is your tank light too bright?
In the meantime you can watch him closely for signs of fin rot and change his water more often, and use Stress Coat when you change his water to help prevent it until his fins heal up.