What Causes Low Resting Heart Rate?

my 18 year old son has a resting heart rate of 45-50. He is an excellent athlete but not necessarily much cardio. He has no symptoms and is healthy. With strenuous exercise he is able to get his rate to 120-130. I am 55 years old and my resting heart rate is about 60 so perhaps there is a genetic component. I also have no symptoms and although I walk a lot I would not describe this as cardio. Thank you