The same thing happened to me a few years ago. I was driving when I felt faint, so stopped the car. I felt sick, so got out of the car and fainted and was unconscious.
I was taken to hospital, where my blood pressure was unusually low and I had every test imaginable.
Fortunately,there was nothing much wrong with me. The official diagnosis was 'vasovagal syncope'. I was told to go straight to a doctor if it happened again. My biggest concern was whether I would lose my driving licence.
If your BIL hasn't had heart and brain scans, I would encourage him to ask his GP to refer him. Hopefully, they will put his mind at rest that there's nothing serious.
Secondly, if he becomes unconscious again, he really should get to a doctor or A&E.
The Wiki page on vasovagal syncope is quite good and consistent with the advice I was given.