Hasn't any of the doctors given him a diagnosis? The irregular heartbeats can be caused by panic attacks/anxiety. I think win could be right. There is also a heart issue called PVC's when one of the valves don't open and shut properly and causes thee hart to flutter for a few seconds. But if he has been seen by some pretty good cardiologists, they should have given him a diagnosis of some sort or maybe he just doesn't want to tell you that he suffers from panic disorder. It can be embarrassing to tell someone that.
We had a pilot that was scared to death to land on a carrier on his first solo. He had to make 5 go arounds until he put the plane on the deck of the carrier and he only did it then because he was running low on fuel. The flight commander had to talk him down. He told the flight commander that he thought his heart was going to bust through his chest. His flying days with the Marines ended on that flight.