It could be that the clean water helped him when you initially put him in QT, and if you didn't do daily 100% water changes while in the hospital tank, it may of continued.
(Make sure it is rot - black edging of the fins, crusty/flaky/shredding. If it looks like a C or a sideways U, it may be him tail biting which is common. Also make sure there are no sharp or pointy plants in his tank, even tiny points.)
If it looks like the pictures below you will want to start an AQ salt treatment. Unfortunately, he will need to go back into his hospital tank, as AQ salt can harm/kill any live plants in the tank.
1 teaspoon per gallon (dissolve before adding fish with it), with full dose of water conditioner. 100% daily water changes for 10-14 days.
If there is no black edging then he is tailbiting and they do that for any number of reasons. Boredom, stress, fins feel heavy. If the filters isn't set on low, or baffled, it could cause him to have to swim harder, which will sometimes make them bite off their fins as to not drag so much.
You said live plants, but if you have any plastic look at them to make sure there aren't any with points on them, or sharp in any way for the fish to snag them on. Even the bushier ones with tiny points are notorious for ripping their fins.
Other then that.. if you can get a picture up so we can varify whether it's rot or something else, it would be helpful. =)