You can keep dosing him until he's better, it shouldn't hurt him, not like salt would. You can also take CJ's advice where you do daily baths for him instead and use a higher concentration for 30-60 minute bath and then just have him in clean water the rest of the time. You can start doing baths next week if you want to try, for now I would just use straight Maracyn until next Friday to see if it helps him more or less. If you see more improvement with just straight Maracyn, keep dosing until he's all better and then dose for another two weeks afterwards.
Rembrandt II, Suiché
Leopards: 1:4 Hawkeye, Luna, Astrael, Dorian, Cullen
Gargoyle: Cloudjumper and Toothless