C.K.
It could be IBS - Irritable Bowel Syndrome:
http://www.answers.com/topic/irritable-bowel-syndrome?cat...
***But it sounds more like Encopresis*** Here are some good links about it:
http://www.kidshealth.org/parent/general/sick/encopresis....
http://www.aboutencopresis.com/
http://www.healthsystem.virginia.edu/internet/pediatrics/...
Poor guy! I hope this diagnosis will help him, and you as his parents. In the interim, keeping extra clothes and accidents as private and discreet as possible is definitely going to help him from any shame, embarassment or teasing he may experience. My heart goes out to him....
Best,
-C.