L.S.
Try to make sure he getting fiber rich foods such as rasberries, strawberries, oatmeal, whole grains etc and pleanty of water. The extra fiber without plenty of water can also slow him down. Prune juice is a temporary fix. A diet high in fiber helps everything go through the system better and helps with digestion. Here is a link that lists all the high fiber foods and how much your child needs.
http://www.kidshealth.org/parent/nutrition_fit/nutrition/...
I hope this info is helpful! L.