First, I would take stock of all the fish in the tank. Are they all looking bloated?
Fast the fish for two days. Give him a chance to poo. If you don't see him pooping, then you should get frozen or canned sweet peas, thaw them (if necessary), take off the bit of skin, mash up the peas and feed to your fish. This should help clear out some of the constipation (if it is constipation). You could also feed cucumber or zucchini.
Next, if the peas don't help, take note of how much the fish is eating. If he is constantly hunting for food, if he's just eating and eating, it might be an internal parasite. You can buy medicines at your LFS to help with parasites, but I'm not well versed in any of these.
Keep an eye on your whole stock. Try to see if this bloated one is eating more or less than usual. Check to see if his poop is white or clear and stringy. This is usually indicative of internal problems.