Is it having difficulty swimming? Sounds like it could be swim bladder? Which Tank is he in? Can you test the water, any other details you can tell us?
EDIT: From the sounds of it Peas may not be able to help him now but here are some treatments for swimbladder from a previous post....
1. Feed your fish a couple of peas. Just get some frozen peas, thaw them, and feed them to your fish. Peas somehow encourage destruction of the impaction. No hard scientific evidence, but it's worth a try. Bloodworms have also worked for me.
2. Fast your fish for a couple of days. Withhold all food for three or four days, and sometimes this alone will break up the impaction and return things to normal. Most fish can go a week to ten days without food and be just fine.
3. Periodic aspiration of the swim bladder works very well. Basically, you stick a needle in the swim bladder and suck out some of the air. Not something to be entered into lightly, but does work well. This is not a cure, but a successful treatment.
Some of these treatments seem a bit extreme but it is worth giving a couple of them a try.