If he is flaring a lot of the time, a betta can blow his tail. The membrane is only a few cells thick, longer fins blow more easily due to rapid movement in the water. My male did this when I first got him. He would be flaring constantly at his reflection and his tail looked like a shredded flag. You just have to keep his water clean until he calms down enough to let it heal.
How long does this usually take, and will it return to it's former glory?