I think that instead of going to the ER and having the same thing happen repeatedly there, it would be a good idea to get an appointment with his regular pediatrician or doctor to talk about this and get a throrough exam.
The talking part, in a non-stressed situation, is important, because you will get a chance to calmly discuss your son's medical history and the possible causes of his problem. You will also have a chance to talk about having him wear a Holter monitor for anywhere from 24 hour to a number of days, so as to catch and record one of these painful events.