How likely is a healthy 50 year old man to die in his sleep?

My boyfriend is 50 and has had a complete cardiac workup done, after chest pains last fall (no cause for the pain was ever diagnosed). He runs several miles 3-4 times a week, doesn't smoke, eats very healthy meals. But he has had a couple of tests that indicate his heart rate drops to as low as 4-5 seconds between beats. He is afraid this will mean he will die in his sleep unexpectedly. I have tried to reassure him but have not been able to find information on the internet to assess the likelihood of this happening. Any information or sources you can point me to would be appreciated!