My 15 year old son, weighing 100 pounds, will experience a racing heart, pounding that I can see it pumping through his chest. It will happen when he is exercising, and sometimes just sitting still. Pain is minimal, no other symptoms. He had and EKG and ECHO in 2013, w/nothing abnormal showing. He hasn t had an episode in a long time, and now recently in last 3 days it has happened 3x. What is causing this?