I have a perc. clown that started breathing heavy. I moved him to my quarantine tank when I noticed a white scale on him. Since then, I have found out that when they are coming down with Ick or some other problem they will all of a sudden start breathing heavily.
If your other fish are not breating heavy, I would move him to a quarantine tank, if you have one, and keep an eye open for Ick or some other problem until he starts breating normally.
I needed to treat my clown with a fish antibiotic (great for sinus infections too). He cleared up in a couple of days and breathing normally. Just put him back in tank.
One other thing, have you done anything to put him under stress??? If you introduce a larger alpha fish into the tank, they will get stressed and breath heavilly.
Hope this helps.