I suspect the boy was saved by the mammalian diving reflex. It is what allows whales to hold their breath for so long. It exists in a much weaker form in humans especially young ones. It kicks in when the face is submerged in cold water. 40 minutes is certainly at the high end but this isnt unprecedented

http://www.today.com/id/36190954/ns/today-today_news/t/mirac...

http://en.wikipedia.org/wiki/Mammalian_diving_reflex