It's being reported already that commotio cordis is the likely cause based on the evidence - per input from medical experts all over the country. Just for clarification, are you making this comment because you believe it's possible it was caused by the booster shot? Or were you just not aware that it's nearly a foregone conclusion what the cause was?
Commotio cordis is the second-leading cause of cardiac arrest in young athletes. It's possible he had another undiagnosed condition, but with all the medical evaluations done at the college and professional level its unlikely a different condition was missed.