To tell if he's constipated.... well, look at the bottom of the tank, and look for fish-poops.
If he's swimming OK, but is simply has an enlarged abdomen, I would simply fast him for 1-2 days and see if the enlarged area goes down in size.
As others have said, you can use Epsom salt. It has a laxative effect, which would help with any bloating or constipation.
Also, what do you feed him? Foods that contain a lot of grains (wheat, corn, soybean, etc) can become constipated. Feeding him a quality protein-based pellet can keep him "regular." The two most often recommended pellets on this forum are: New Life Spectrum pellets and Omega One Betta Buffet pellets.
Personally, if this was my guy, I would simply fast him for 1 or 2 days, and then switch him over to a good protein pellet (such as New Life Spectrum or Omega One Betta Buffet pellets).
However, if you want to use a low dosage of Epsom salt, it won't hurt any. You can get this from supermarkets or pharmacies. (Look for 100% magnesium sulfate. No dyes, perfumes or other additives.)
Put some of his current tank water in a conainer. Dissolve Epsom salt at a dosage of 0.5 teaspoons per gallon. (A 5 gal tank would need 2.5 teaspoons.) Stir until the salt is completely dissolved. Slowly add the salt to his tank, over the course of an hour or more. (Slower is always OK.)
Salt doesn't evaporate, so you do not need to re-add it. And it's OK for him to remain in it for several days.
When you do your usual water changes, just use regular conditioned water (with no Epsom salt). And you'll slowly remove it from the tank.
If you want to remove it at a faster rate, just change out a larger volume of water. Replace it with regular conditioned water (with no Epsom salt).
On the other hand, if you feel the Epsom salt isn't having enough of a laxative effect, you can increase the dosage. MattsBettas recommended 1 teaspoon per gallon, and that would be a good dosage, as well.