Definitely isolate him. Not only could he infect other fish but fishes sense when another fish is injured and they will often attack a disabled or injured fish. In a "hospital" tank, start a course of a good antibiotic (Kanacyn is absorbed by the skin). And keep him clean (usually meds say to do the whole course of treatment for X number of days before cleaning the tank but I do daily cleanings anyway which seem to make the fish feel better since antibiotics will kill beneficial bacteria in the tank and on/in the fish too, thus lowering your fish's natural defenses). It is not unusual for a fish being treated or immediately after treatment to come down with a secondary case of something else--so I keep the hospital tank clean and just redose after cleanings. Make sure you remove the carbon from your filter--keep a filter running in the hospital tank since many antibiotic deplete the oxygen in the tank, remove live plants (substitute fakes to make the fish feel more secure), remove any inverts too since antibiotics will kill them.
If the sore gets worse, you can use peroxide but that's an extreme measure.
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