That sounds like something out of a horror movie, there are things moving in his skin! It sounds like some sort of parasite thats gotten inside of him. I could be COMPLETELY wrong but can only assume that there are two possible situations that will occur. 1: He cant fight it off, he dies, and you only hope it doesnt spread to the other fish. 2: its not that big of a deal, it goes away, he lives to tell the story! I'd say the only thing you can do is move him to a seperate tank so it cant spread to the other fish and you can add a small amount of copper (ask about that at your
LFS) and hopefully that will help.