Honestly, back then, had I known that "leaving them alone" is sometimes the best treatment, I would have. I have had zero luck w/ treating fish in QT, and I've treated 2 fish in qt, both died in treatment -- but both were eating heartily in the DT. You'd have to treat ALL The fish and leave the tank empty for a few months, otherwise they get it once they go back in the DT. If yours is eating, you will just have to hope he fights it off.
Yes -- qt'ing from the start pays off because no matter how stressed your fish get, they can't succumb to parasites that are not in the tank.