My son has stomach pain...

There are several conditions in the digestive tract that can occur as a reaction to food intake and the tests they run are typically done when the stomach is empty. The other area you may want to look into are the interstitial lining and any disturbances there. In addition a structural abnormality can be occurring in the esophagus that will not be seen in traditional testing. I would recommend I do a scan of him that will give you more specific insight.