Describe exactly what is growing on him. Is it white spots? Fuzzy? Does he look velvety or like he has a fur coat? Is it little stringies? Is he bumping into stuff and trying to scratch himself?
If you don't know how to describe it, here's a link with a few pictures and detailed information of how to help it. Though, don't assume the worst. He probably just has an infection or a fungus, at most. Don't think that it's fish TB immediately. Treat first, then diagnose (as Dr. House would say).