Dear Joey,
Of course, I am not able to tell exactly what is going on with your son without looking at his studies and evaluating him directly. Therefore, it is unclear whether this is an actual arrhythmia (abnormal heart rhythm) or just due to dehydration, which is much more common. The first thing that I would start with, though, is to make sure that he is adequately hydrated. He should have four 8-12 ounce glasses of fluid every day and a salty snack, he should not skip meals, and he should have no caffeine. Unfortunately, the sensation of his heart going fast because he doesn’t have enough fluid on board is scary enough that it can perpetuate the symptoms for a while.
If these do not make these symptoms go away, you should probably see a pediatric cardiologist. There, the evaluation that will occur will likely include an ECG and consideration of a Holter monitor, which is a 24-hour monitor that records his heart rhythm continuously. If he is having his symptoms daily, it should capture his heart rhythm and help diagnose his problem.
Im not a doctor. While in the ER, did he have an EKG done? If not, order one and go to a pediatric cardiologist. Record his episodes and tell the DR.