C.L.
I would take him to a doctor. He could get dehydrated. He might have a virus (I heard rotovirus is going around), parasite or a food intolerance or allergy. You also might want to take him off dairy products and try the BRAT diet (ripe Bananas, Rice, Applesauce and Toast). Too many fruit juices can be a problem too. Keep him hydrated with water and Pedialyte. Good luck!