[physiobob]

Sounds like referred rib pain, common in running. If he has been cleared via ECG then I would work on his thoracic mobility especially in rotational movements.

Perhaps he can also do us a favor and also monitor his HR when he gets the pain. If there is a consistency to that then if he can try and achieve the same levels on cross trainer and/or stationary bike and let us know if he exhibits any symptoms with that exercise.