Lower the depth of the water so that he doesn't have to swim such a long way to get a breath. If you haven't already, add some tall, big-leafed plants that he can rest on, too, as this will make it easier on him. Maybe a bit of aquarium salt.
How old is he? Is he displaying any other odd symptoms?
Edit: I just saw your other topic. Is this Finley, the fish you referred to earlier?