Brief Answer:
Several possibilities

Detailed Answer:
I read your question carefully and I understand your concern.

Judging from the history you provide the most common explanation for those episodes of loss of consciousness and shaking would be that of seizures.

Seizures though can be due to many causes. I see from an other question you have asked that you mention he has done MRI, I assume of the brain. That is a good thing as it does exclude brain lesions like tumors or stroke which are the first to be thought of when there are seizures appearing for the first time in an adult.

However even if brain lesions are excluded there are other situations where the brain can suffer due to external factors and cause seizures. One possibility would be metabolic alterations like blood glucose or electrolyte abnormalities, but I assume he has already had some routine blood tests to check for those since he has had an expensive test like MRI.

That leaves another scenario which might be insufficient blood and oxygen to the brain.
Insufficient blood supply is commonly caused by heart issues such as abnormal heart rhythm. For that reason prolonged EKG monitoring is first advised to check for episodes of abnormal heart rhythm which might compromise blood supply to the brain. If no heart related cause is found imaging of the neck vessels might be considered as narrowing of neck blood vessels may lead to insufficient blood supply to the brain as well.
On the other hand if no such cause is found there is also the lung issue. Advanced lung disease may cause low oxygen in the blood, as a result the brain does not receive enough oxygen and may cause the described episodes. For that reason if no heart related cause has been found a consult with a pulmonologist is needed to review his treatment and also to consider respiratory directed physical therapy (as already suggested by a colleague on this site).

I remain at your disposal for other questions.

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