When I got my betta fish from petsmart, I thought I saw a very tiny hole in his tail, but otherwise he looked great so I got him because he was very interested in me and his surroundings. I thought it may have been the shape of his tale but I have looked very closely at him since then and he does have it. I read that it should heal up on it's own if he's in a better place to live than the cup they come in, but aquarium salt is supposed to help too. Though I'm not sure if you should put the aquarium salt in if you have other fish in the tank as well and depending on how bad it is, you might need to use aquarium salt instead of letting himself heal. Hopefully someone else can help you with that....I hope I helped some! :)
I have a plastic plant in my tank too but i'm planning to switch it out if anything goes wrong, i'll probably switch it out anyway.
But I hope your betta gets better soon.