So sorry for your loss. There are a couple of possibilities here. The issue is that your son's heart became enlarged, to understand what happened they would need to determine the root cause. The two types of enlarged hearts are hypertrophic which means the walls were thickened and is usually due to high blood pressure. The second is dilated cardiomyopathy which the walls are stretched out and is usually due to a previous heart attack(s) or a virus. In either case the muscle becomes too damaged to contract normally and it stops beating.
I hope his doctor can give you some closure.
I'm so sorry. What a terrible thing to lose your son at such a young age and without any warning. Please accept our condolences.
This is not going to help you very much, but by way of explanation, this happened to a friend of mine. Luckily he was in the hospital at the time. They decided to stop his heart completely and then shocked him back to live to a normal heart rhythm. He is ok now, but it goes to show you how much is needed to control this situation.
You are not alone in being stunned by this terrible thing that happened without warning. Surprisingly often, the condition that took your son has no signs or symptoms; the heart just suddenly develops a crazy rhythm that is not compatible with life. There is very little that could have been done.
Here is some more information about this fairly rare condition:
http://abcnews.go.com/Health/t/story?id=7422278&page=1
I am very sorry for your loss.