Two words for you: Potassium Permanganate.
Grab a white bucket fill it with two gallons of distilled water and add enough potassium permanganate to make a strong purple solution. Then dip the affected fish for 30 minutes. Monitor the fish to make sure he's not too stressed. If the fish shows any signs of stress, put him back in the main tank and try again the next day, but this time with a pinkish solution. Do not use tank water for this treatment, nor dechlorinator until you are done. Dechlorinator will make the potassium permanganate ineffective.