Not sure where you are in the application process (current year candidate or a year plus away) and what if anything DoDMERB has been told. I am not a doctor, but it sounds like IRBBB is a symptom of other potential underlying conditions. What I can tell you is don't list on DoDMERB that your DS has IRBBB until the cardiologist confirms it (or at least seek their guidance on how to complete the form.)
Many people have put down on the DodMERB form that they were at one time diagnosed with something when follow-up visits actually rule that initial/potential diagnosis was incorrect. But what happens if you put it on the form, it can be disqualifying and you spend a great deal of time explaining to DoDMERB that the initial diagnosis was deemed to be incorrect.
Doctors may run a lot of stuff to rule things out and they may even list an initial diagnosis to justify the tests, but this doesn't mean the diagnosis is correct.
I am in no way suggesting that you hide anything from DoDMERB, but while your DS is still being evaluated - don't rush to claim that a diagnosis has been made.
Hopefully, your DS just had a bad day and passed out. (My DD had a single fainting episode in high school - (she is now at USNA) she listed it and that a doctor evaluated her and it was just a combination of being in a hot church, poor hydration, and standing up too quickly)
I hope and pray your DS has no serious condition, but I do caution you on jumping the gun to report to DoDMERB until whatever it is - is confirmed and it appears you are seeing a cardiologist to get a final confirmation.
(Sorry if this appears rambling, but the DoDMERB threads are full of examples of people trying to explain away an initial diagnosis that proved to be incorrect)