when i was a child, my heart was sometime pain after big laugh. Then i need to stop breathing a while. Then no painful. One time, I shared the experience with my classmate and she said she had had the same situation.
that time, i had't told my mother this painful to prevent her worry.
when i grown up, no this experience happen again.
i think you should bring your son to doctor there to have a check in case you are worry about it.