I don't claim to be a disease expert but I do know a little about lionfish and prefer to be proactive, because the more time goes by the more risky a bad outcome occurs. My 1st line of defense is perfect water quality and live food. If you can move him to an observation tank alone with optimum water quality that would be best. If he can not be moved post your tank situation, size, reef or fowlr, tank mates. The nutrition of live food can not be rivaled and contains so many things that alone with quality water heals many things even without medication. In this case I would treat with metronidazole, it's a relative safe medication that lions tolerate well. Live food recommendations include ghost shrimp, guppies. and mollies; fish are the best complete package with live gut flora that is invaluable.