I've replied on other threads, and realize some here are not on the premium board, so I'll link this:
https://www.ncbi.nlm.nih.gov/books/NBK526014/Commotio cordis is the most likely scenario here. He tackled, fell, got up, wobbled, and then went down.
The best thing for him is prompt CPR, which he received. They also used an AED on him, implying that he had a shockable rhythm, pointing again to a cardiac rhythm problem such as described above.
Given that he had a return to spontaneous circulation and is stable though critical at the hospital, the biggest question is his neurological status. Typically this will take a couple of days to determine, as they will rest his brain with sedation to give it time before attempting to wake him.
The next question after that is whether he has some sort of condition that primed him for an arrhythmia, such as long QT syndrome, which can be present in otherwise completely healthy individuals.
Extremely rare and will be the subject of papers and in textbooks in the future.
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