Hi A.,
I'm not a doctor or anything, but I would have his liver checked. Sometimes protein digestion problems can cause a protein-style malnutrition. Fats exit into the bloodstream near the liver and protein carriers are needed to transport them away from the liver to other parts of the body. When protein is drastically unavailable (is he having any growth delays?) fat accumulates around the liver and it looks like a pregnant belly.
That's one possibility. A distented belly can also be an intestinal worm, but that is unlikely in America. You'd have probably known that by now.
Either way, you're right to be vigilant and should have that checked out.
Good luck.