Actually, it looks to be either tail biting or he tore it up on the plastic plant -- I can't see much as far as fin rot, and that much damage in two days is more likely caused by tearing or biting.
I would pull the plastic plant, personally -- maybe replace it with something softer. Otherwise: keep the water clean and feed a high protein diet and it should grow back. If it's tail biting you may have to put more work in though. Common causes are boredom, stress, or even sometimes "just because" or to deal with heavy fins.
You can try and see how well it starts to regrow after pulling the plant and keep an eye on it; if there's nothing sharp in the tank for him to be tearing his fins on, and it gets worse, then odds are he's tail biting.
ETA: Actually, looking at the After pic a bit more, if it's not just the picture and his anal and dorsal fin are affected as well, my money's more on the possibility that he's biting his fins.