J.F.
My son has the same problem. After a year,I finally went to my pediatrician and found out it can be a medical condition.
See the attached web -site http://familydoctor.org/166.xml
I had to put my son on a fiber supplement after clearing his bowels with an enema. We are still working through the problem but it has gooten a lot better, now that we know how to help him. They hold their bowels, until they get constipated, then when they try it is painful, so they hold it more, thier bowels get stretched, they leak poop and don't even feel it. It's a horrible cycle. What starts as a behavioral issue, becomes a medical problem. After you get rid of the impacted stool and give the bowels time to come back down to size, you can start to retrain the behavior. My pediatrician says it usually takes about 3 months. We're in month two right now.
Hope this helps.
God bless!!!!