...That doesn't look like biting to me. Usually when they bite, they get more towards the top of their tail and their dorsal fin, but the damage your boy has is at the bottom of his tail and on his anal fin. With that said, it doesn't look like finrot either....it looks to me like his fins got caught in the filter intake or something.
All those plants I'm seeing in the tank also look rather sharp...I'd suggest pulling those out and replacing with softer silk plants, and perhaps covering the filter intake with a sponge or a bit of pantyhose.
Yes, salt and clean water are definitely a good way to start, as mentioned previously...up your PWCs to a couple times a week(assuming this tank is cycled), but remember to only add salt equivalent to the water you take out.
However, that patch worries me....it looks like it might be something bacterial. Does he have any other off symptoms? Is it eating? What are your water params? Is the patch fuzzy at all?