Don't treat a Tang with Copper. They have a more delicate system than many other fish, and copper does intestinal damage to Tangs.
If you can remove him to a hospital tank, that's an option, then he can be treated with hyposalinity.
Your fish does not look good in that picture, time is of the essance.
Read this here:
http://www.reefs.org/library/article...tes-jorge.html
and this...
http://saltaquarium.about.com/librar.../aa102797d.htm