Abdominal pain and arthralgia in 13 YO Male

My 13 YO son has been ill for 5 months now. It started as what we thought was mono but quickly ruled out. His symptoms are upper stabbing ab pain (both sides, below the rib cage), no intestinal disruptions, frequent illnesses including sinus infections, ear infections, arthritic pain in his ankles and hips only (intermittent), low-grade fever (intermittent), extreme fatigue. Appetite - ok. He has not lost any considerable amount of weight and has had upper/lower GI, abdominal CT & sonograms, seen a GI doc, a hematologist and an immunologist, who did not find anything significant. He goes to school everyday but is always feeling horrible; he's just gotten used to the pain, which he says varies from a 6-9 (on a scale of 1-10). The only trigger that he seems to have for the pain is exercise, so much so that he had to give up baseball which was his favorite sport. He is an honor student and a laid back kid. Help! We are all so frustrated, not sure where to turn. He was tested for Celiac Disease and all the other possible intestinal disorders but nothing showed up. The only thing that we have not done is allergy testing (he was tested 6 years ago and had no reactions) or a fecal specimen to rule out possible parasites. Any other ideas?