With the way it looks, I'd be inclined to say it might be tail biting. I've got a tail biter at the moment and the photo looks awfully familiar. Watch him for a while and see if you can spot him doing it. If not, it might be finrot. Do the boarders of the ragged fins look black at all? Either way, make sure the water quality is as good as it can be, he's and that he's not getting bored or stressed. How long have you had him for? Has anything changed for him recently?
Take the plastic one out. I had plastic ones in with my betta and it shredded his tail. I then bought Bettafix and treated him with it. His tail was good as new in a week. Plastic plants can have some jagged areas. Its best to use real or silk plants. I have silk plants in with my boys now and I havent had any more fin injuries.He has one plastic in there and a nice sized real one