Hi. Many things can cause passing out. Seizures is one of them, but diabetes, hypoglycemia, and heart problems can also cause fainting.
Has he ever been tested to see if he has seizures? If he has not that is one avenue I would check into. The EEG which is the main way to see if there is any seizure like activity going on does not take long and is painless. I am just throwing that out there as something that should be ruled out.
I hope the best of luck for you.