He may have something wrong with his GI track so your general doctor may have him go see a specialist where he will have some lab testing done. I went through the same kind of pain on the right side of my lower abdomen. The doctor sent me to a gastrologist turned out I had Chrons so just be wary. Try having him hold a heat pad against his stomach to relieve pain, helps me when I have flare ups. Good luck and hope he gets better!!!
on a woman i could tell u it could be a cyst, i have no idea on a man. but it also can be scar tissues causing that and it be nothing but more surgery if it is. hope this help u out here.