Haven't had this experience yet. But if it were me, I would try to explain that the food goes through the body and needs to come out or else our tummy will hurt. Not sure if you can find a book about it and read it to him. Maybe he's really uncomfortable with the feeling of poop in his diaper...it might be a sign that he's ready for the potty. Maybe next time you see him holding it in bring him to the potty and try on there. Hope that helps!