If he's hovering by the surface, you should definitely check your pH value! A sudden crash in the pH would make your fish stay motionless by the surface, and soon get milky skin. If your pH is below 7 (goldfish prefer 8 actually) go to your pet store and buy some pH buffer
Change the water once a week to keep the KH value under control, this will automatically keep your pH at good levels!
If he's gasping, or simply breathing by the surface, your water might not have enough oxygen. Do you have a good filter? Any airstones?
But if the other two fish aren't doing anything like this, it might be good to have a close look at his gills! If there may be parasites or something bothering him. Try looking if the gills have healthy, meaty color, or if they're brownish or grey to the color. Any holes or wounds on the gills and gill covers?