What Causes Headache, Stomach Pain And Vomiting?

My son is 12 and 2 months ago started complaining of headaches, stomach pain and vomiting. He's has all kinds of tests, MRI Brain scans, blood work & all childrens's hospital can't find anything wrong. In 2004 he was hit in the head with a baseball and knocked him out cold. Now for the past few months he's been sick, throwing up all h\the time. HELP