This is something to ask his doctor. You can do that, have the doctor explain exactly what happened and why. It may have been an arrhythmia problem where the heart wasn't beating normally. In some cases, especially younger people, that can cause death. Or an arrhythmia can cause blood clots which will cause a blockage and heart attack. Valve problems, obesity, family cholesterol problems can all lead to heart attacks. Only the doctor knows for sure what happened and why.
I'm so sorry about the loss of your son. I can't imagine the pain you're feeling right now.
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