My eldest son, who is now 14 had this problem when he was little and it reoccured until he was about 7 yrs old. It's quite common in boys but it can end up ruling a parent's life!! It has a clinical name which is 'encopresis.' It starts because for one reason or another your son doesn't like having a poo and is too young to know that he must have one or he'll be ill and blocked...so he thinks if he holds it in then he'll never have to have one again. What happens is that it does find away out and then obviously smears on underpants etc on a very regular basis or becomes hard and hurts! The pressure can then numb the nerves in the anus, so then he doesn' t actually feel that he needs to go anymore!! i found that the doctor did not take it very seriously, whilst I was tearing my hair out. After much research on the internet, I decided that as it was going to worm it's way out into his undies, then if I gave him a laxative, then he would have to get to a loo at some stage because he wouldn't be able to hold it back...the feeling would be too strong. I gave him lactulose, which is fine for kids. 2 teaspoons twoce a day...and it worked. Once he got into a regular pattern, the cycle was broken. He had some reoccurences but I just did the lactulose trick. I can tell you that it is a damn nightmare and the whole day was dictated by pooing or not pooing!! The laxative really does work. Good luck! x