Welcome to the site
As Melosu stated, check for white spots that look like grains of salt or a dusty covering. Also is it rubbing against the rock or anything else?
If he isnt in a QT already I would recommend getting him into one to keep a close eye on him and possibly treat, once we get a better idea of what it may be. I'm dealing with a fish now thats having some rapid breathing, theres multiple causes, so we'll just have to try and rule out some things.
some things that cause rapid breathing - ich / velvet ( white spots)
( with the above and below possibilities is he rubbing on anything?)
gill flukes / parasites - can cause rapid breathing, may or may not see 'worms' attached to the gills
bacterial infections ( any other signs?, hard to say for sure by the looks of the picture) may show deteriorating fins, reddness on body etc.
let us know..