Unless you set up a QT tank with cupramine, all you can do is continue feeding and keeping him healthy, and he will naturally fight it off (hopefully).
My tank got hit with ich a few months ago, and it would have been impossible to remove all the fish without a complete teardown. Everything, tangs included, survived by keeping them well fed, and soaking mysis with garlic. Several garlic drops in the tank every 2-3 days also seemed to help keep them wanting to eat. Several small water changes help a bit too.
This has worked for me twice in a 125 gallon tank. Although ich is still in the tank (unseen/dormant?), the fish are much more likely to survive after beating it once. My yellow watchman has gone through several tank outbreaks over the years in 3 different tanks, and never got spots again after surviving the first bout.
This doesn't work so well in a small tank though. I believe volume of water is a big factor.