My guess would be biting, honestly! I've had biters in the past and some of them would literally eat their fins overnight! Keep the water clean and watch out for fin rot. If he has fin rot there will be black along the edges of his fins, but if it's clear edges then it's regrowth.
I had a betta named Mook that was such a brat about nipping his fins! I never got them to fully grow out because he wouldn't let them. When he continued I would rearrange his tank, add new items, take some out, etc. I ended up giving him to a woman with a larger tank that was in a busy area of her house because I figured out Mook was just bored. Apparently eating fins is fun!
I'd try rearranging items, adding something new maybe, or even finding him a floating toy!