If you are tall it takes longer for blood to reach the brain – so you feel faint more quickly. I passed out 3 or 4 times (once down a flight of concrete steps) before this was explained to me. I now never drink alcohol without a glass of water and a full stomach. I am also really conscious of hot environments (gigs / pubs etc) as this has been a factor every time.
I already eat more than anyone I know, so that was not the problem for me, but sounds like it might be for your son.