Actually this is normal in person who are athletic like you son. I believe that the heart gets a bit enlarged, due to what is called cardiac hypertrophy. The left ventricle of the heart on systole, pumps the blood out from the left side of our heart. The left ventricle is located at the bottom portion of our hearts.
What happens is that from excessive exercise from activities like sports, the heart actually get so conditioned from the constant level of exercise, that someone like your son will develop a slow heart rate, called Bradycardia. Actually this can be very beneficial and people with this condition usually live a long long time.
So in essence this condition is not pathological in nature. In other words there is really no need for concern. Hope my explanation of this syndrome helps you out.