With tangs its hard. If you're able to get him in a hospital tank ( you can set one up pretty quickly if needed, just need to do more water changes) and do hyposalinity or copper is going to be your best bet. As stated before, 'reef safe' treatments dont work well.
This is why its so important to have a hospital tank before putting your fish in the display! It saves you a lot of hassle, stress and possibly heartache later on.
75 Gallon Saltwater Reef
Just your average LVT (licenced vet tech) trying to help the world's animals!!