Suggest Treatment For Rapid Heart Beats

Hello, my 13-year-old son has been having a very rapid heartbeat in the past 3 days: 130-147. It is possible that he has been having this problem for a long time and we have not notice. He exercise a lot and even lifts (as much as his own body weigh). Also he is a vegetarian, so we suspect that he may be anemic. We discovered this problem 3 days ago during a preventive check-up. His heartbeat was 120 and the doctor said that his EKG was ok, so that it was probably just caused by his emotional distress. Yet we have measured his heartbeat in the past 3 days at home about 8 times and it is never lower than 130 and never higher than 147. He feels okay subjectively. He runs up and down the stairs as an exercise every day and has no trouble breathing. He has increased his body weight (especially muscles) in the past year. We are traveling right now but we do plan to see his pediatrician. I am just very nervous and would like to know whether it is an emergency or whether this can wait for a week or two. Thank you.