WHATEVER YOU DO, DO NOT PUT HIM DOWN.
FIrst things first, the disease will be away from the fish, but it will still be in the tank whcih means the other inhabitants will still have a chance of catching it. Second, putting down a fish is a cowards way out. And third of all, it looks like fungus and fungus isnt to hard to cure. Just buy some medication at your fish store (there common and not expensive) and treat accordingly as the bottle says. Make sure you remove activated carbon while treating a tank with medication