Well, if you can keep him in a QT untill he recovers, then that would be perfect, I would recomend droping the salinity in the QT, and leaving it low for a wile, then raising it again, BUT leaving him in the QT for quite a wile after he recovers, so when you put him back in the display he wont be reinfected (since treatment is stressful). Also since you are worried about Cu, I would recomend using a non-Cu treatment, I'm not sure which ones are Cu free, but it would be your safest option... possibly also use a methiline blue dip? (not sure on that one, have heard good and bad things about it).