My 9 Year Old Son Is Has Been In And

My 9 year old son is has been in and out of ER's lately for severe abdominal pain, accompanied with dizziness, headache and nausea. The main complaint is stomach area and it was thought that he was having a bout with his already diagnosed chronic constipation. All of his tests from CT scan of abdomen, upper gi and blood and urine tests as well as CT scan and EKG are normal! He is now scheduled for an endoscopy and colonoscopy. What could this pain be? It has caused his otherwise huge appetite to diminish. Also, pylorus looked narrow but upper gi view looked normal.