Just to add for your information, if it is cardiac (heart), the sooner it is treated the better.
If there is a blockage to the blood vessels that bring blood to the heart muscle, this will cause pain, (like your arm going to sleep after lying on it), if it continues, the muscle will die from lack of blood supply. This will lead to that part of the heart not working. Depending on which part, the heart can go on functioning or be badly impaired.
If clots are treated early, this damage can be completely prevented or minimised. Unfortunately once the musle is dead, there will be no pain from that area anymore, so you don't know it's damaged, but the heart will function less effectively. An ECG will show any damage.
If it is not his heart, at least his mind will be put at rest (and yours) because stress will do no one any favours. Please get him checked, especially with his family history - at the very least he will have a check of his current heart status. How old is he?
Best wishes