What Causes Pain In Upper Back, Torso And Stomach?

My 10 year old son for the past 8 weeks has had random severe upper back pain, front torso and stomach pain. He has been tested for inflammation, lupus, etc. Stomach ultrasound came back normal. Chest x-ray came back normall. All the blood tests have come back normal. He cannot attend school as sharp random pain comes making it impossible for him to concentrate and sit in school all day. Any ideas would be appreciated.