Make a diagram of the large intestine, showing large "golf balls" of stools at the end of the large intestine. Show your child that voluntarily holding onto his stools makes them harder, and that is why it hurts to pass them.
And we also used a glycerin suppository (another recommendation), which had good results.
Later in the evening, my sweet MIL called and I told her the story above (only with more detail). She exclaimed happily when I mentioned the glycerin suppositories because, as she put it, she had forgotten about those. She thought they'd be the perfect solution to her 86 year old mother's constipation as well.
So, was Capt. Adorable's constipation a coincidence? Or was it fate that he'd be constipated and I'd research the remedy and tell sweet MIL about it?