Get him into a QT and start treatment. Check out the many, many threads on here about how to treat ich. How long have you had him? Who else is in the tank? Chances are the other fish are also at risk too. Don't fall into other treatment types, only two proven methods, copper and hypo, both to be done in a QT followed by leaving your tank fallow for up to 6weeks. This ensures the parasite can die off without a host (fish)