If you can remove and treat I would recommend Maracyn 2 or you could try treating the tank with Bactershield.
Looks like uronema a type of parasitic infection with the sores. Suggest quaranting all fish and use chloroquine phosphate. That should start turning him around rather quickly. Did you add new fish to the tank?
+1.you can add metronidazole into his food. Mix general cure by api