there may be two reasons: one reason would be that he is in great shape. runners tend to have slower heart rates (50-60 bpm) so if he is a runner in good shape this could account for a slower and stronger heart rate. unfortunately another reason could be a hidden cardiac problem. i don't know how slow his heart rate is or if his heart rate is regular skipping beats etc so it would be wise for him to have a sports clearance by his medical provider or even a cardiologist. this would be for your son's own protection.
why would a 13 yr old athletic male have a slow heart rate