P.G.
Doctor. Now. It's common, but it's not healthy. It's constipation - encoperisis. It happens for school age kids. Stop getting angry at him and believe him. Like Retta said, if it's bad enough, they leak, and they can't feel it. It's treatable with miralax and diet change, and some "retraining" - basically a more structured bathroom routine (for us it was 5 minutes on the toilet right after school/camp) to get the body back into routine.
My son eats a lot of fruit and a reasonable amount of veggies, but kids don't like to poop at school, camp or whatever, and they hold it. He even got a UTI from the constipation. X-ray showed he was backed up to the middle of his large intestine.