Brief Answer:
Several possibilities.

Detailed Answer:
I read your question carefully and I understand your concern. I take it your son was alone and there was no one with him to describe what happened (any motion of head or limbs, eye deviation, skin pallor or cyanosis, sweating, pulse etc).

In that case there are several possibilities to be considered. First possibility is a transitory heart arrhythmia which may lead to a sudden drop of blood pressure. As a result there is lack of blood supply to the brain and loss of consciousness.

The other possibility is a seizure, loss of consciousness due to abnormal electrical discharges in the brain. At this age a seizure usually doesn't occur out of nothing though. It can be due to a brain lesion such as stroke, infection, tumor etc (usually should be some other symptom though esp in the first two cases, such as headache, weakness of the limbs on one side etc). A seizure can also occur due to alcohol (consumption or withdrawal) or drugs, metabolic changes such as low blood sugar, dehydration, electrolyte imbalance etc.

So assuming he is feeling well now an evaluation is advised anyway. Initial routine tests would be an EKG and some routine blood tests (blood sugar, liver and kidney function, electrolytes). I something is found it will be addressed accordingly. If they result normal and there is some issue present last night such as alcohol use, dehydration etc it may be left at that. If no such issue is involved then a neurological evaluation for unnoticed neurological signs is necessary and should be considered the possibility of scheduling an EEG and head imaging CT/MRI. If there are neurological signs naturally those tests should be done on an emergency basis.

I remain at your disposal for other questions.

---