You took your child to the doctor and the doctor didn't see an "anything wrong" with them having fecal accidents in their underpants??? Sounds to me like looking for a new pediatrician is in order.
A child can leak entire bowel movements without knowing it. Take them to a pediatric gastroenterologist that can check them for an intestinal blockage.
Please don't punish and humiliate your child for what sounds to me like a medical issue. Let me ask you a question. Does your child lie to you about other things--consistently? Why would your child lie to you about this unless it is because he is afraid of your reaction so he hides it!
It is frustrating and that is why your first point of contact should be with a pediatrician that believes that when a child sh*ts his pants, it is an issue!
Read this beginning to end!
http://www.med.umich.edu/yourchild/topics/encopre.htm
This link has some recommended kid friendly books and videos:
http://www.med.umich.edu/1libr/guides/encopre.htm
This is from the NIH:
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002537/
Many kids that "suffer" from this condition actually need to see a psychiatrist because of the negative self esteem and social issues it can cause. "Up to 20% of kids with encopresis experience feelings of low self-esteem that require the intervention of a psychologist or counselor."
http://kidshealth.org/parent/general/sick/encopresis.html
Be your child's supporter--not tormenter. Punishing them won't work for a medical issue he has no control over. Again, any pediatrician worth their weight in salt would have checked this out! Constipation is a very common problem in children and typically the "blockage" can be felt in the lower intestine!
"problems with encopresis and constipation account for more than 25% of all visits to pediatric gastroenterologists (doctors who specialize in disorders of the stomach and intestines).
http://kidshealth.org/parent/general/sick/encopresis.html
You don't even have to "read" on this link...you can listen by clicking the speaker! How simple is that!
http://kidshealth.org/parent/general/sick/encopresis.html
I would read the above, educate yourself and then tell them you are "sorry" for your behavior--that you love them and you will work through this as a "team". Don't punish them because you can get through this! It's a long road and believe me, positive encouragement works better than negative unless you want to deal with depressed teens with low self esteem.
Feel free to email me for emotional support because you have a "friend".
Talkstotrees is correct:
"The colon's job is to remove water from the poop before it's passed. The longer the poop is stuck there, the more water is removed — and the harder it is to push the large, dry poop out. The large poop also stretches out the colon, weakening the muscles there and affecting the nerves that tell a child when it's time to go to the bathroom. Because the flabby colon can't push the hard poop out, and it's painful to pass, the child continues to avoid having a BM, often by dancing, crossing the legs, making faces, or walking on tiptoes.
Eventually, the lower part of the colon becomes so full that it's difficult for the sphincter (the muscular valve that controls the passage of feces out of the anus) to hold the poop in. Partial BMs may pass through, causing the child to soil his or her pants. Softer poop may also leak out around the large mass of feces and stain the child's underwear when the sphincter relaxes. The child can't prevent the soiling — nor does he or she have any idea it's happening — because the nerves aren't sending the signals that regulate defecation (or pooping)."
http://kidshealth.org/parent/general/sick/encopresis.html#
Before your next medical visit, things you can do to help him: Increase water substantially, eat a high fiber diet, and have him sit for 5-10 min after meals and before play.