He uses his diaper at the most inconvenient moment.

Jim fiddled with his hands slightly, his eyes twitching and his hands both out of an internal debate of whether he was going to do and and deciding what to say. He decided that he was goign to talk to her. That this time, he was going to invite her to play with them and maybe ask her to be his girlfriend.

Then he felt a familiar sensation in his body.

"Uh oh."

While Jim was not incontinent, he had lived in this town since he was a baby. While his parents had potty trained him to an extent as a precaution should they ever leave town or if they run out of diapers, Jim always had the pavlovian instinct that whenever he wears a diaper, he is sure as hell going to use it.

So when he felt that familiar odd rumbling in his lower tummy, he already felt the back of his diaper become hot and squishy, the waste rushing out of his body nice and easy. He also felt the front of his diaper harden into a wet lump. Having lived their whole lives in dirty diapers, Jim and Timothy almost didn't notice the poop in his pants.