My Son Who Is 12 And Healthy Ate His Breakfast

My son who is 12 and healthy ate his breakfast then approx 10 mins later got up to get ready for school and passed out for 10 seconds max. We took him to a&e and they discovered a prominent rsr pattern in V1, referred him to the cardiac unit but said he was fine to go back to school and carry on as normal until we get the appointment. I'm worried sick that he may have something seriously wrong with him and don't know how long I can wait with the fears I have. What could it be? Is it life threatening?