If your filter intake isn't covered, its very likely his finds got caught in it and got ripped up. He also could have just bitten himself silly for some reason, or maybe a little of both. Its hard to say which exactly.
Cover your filter intake with a bit of panty-hose and a rubber band or some filter sponge if you haven't already and just keep up with your water changes and watch his fins to make sure fin rot doesn't set it.
It also certainly wouldn't hurt to add in a few more plants for him to swim around and hide in either when you can. Bettas come from a densely-planted natural environment, they feel a little more secure when they have a decent amount of cover.