this sounds like he had a reflex anoxic shock seizure. please DON'T panic at what I have said.....this does NOT mean he has a serious disorder...just that he has had a reflex reaction to a big shock.
My DD3 had one once, from trapping her thumb in the hinge of a door. She was unluckier tho as she was out for over 2hrs, and then in hospital for a few days under obs (she was 23mths old). DS1 also used to breath-hold after a shock bringing on the same reaction....(I got an odd bunch of kiddies that like to keep me on my toes[despair emoticon]!).
Now should he do it again then you may want to get him checked out, but 99% of kiddies never do....it is a one off bought on by shock, like your son!
I can't do links, but there is a website which can explain more than I can.....
www.stars.org.uk/
please please don't read too much into everything written here tho.....this is an organisation for the 1% of kiddies that have it bad. Like I said, your son will MOST LIKELY be the 99% of ok kiddies that have it once and never again (like my DD3 did. DS1 had more, but from the age of 4 he hasn't once had an episode, and never never did at school either!)
Doesn't stop you from fretting and remembering the terror tho.....you have my sympathies for that. I will NEVER forget how grey DD3 went the day she had hers, and how terrifying it was when they couldn't bring her round. Alternitively, DS1 only ever went blue, fainted, then came round again. scary, but never needed an ambulance. And after the first couple of times I got so blase I never even got him checked by anyone.
anyhow.......good luck tonight and rest easy (if poss) and read the stars stuff when you are calmer.
HTH....and take care of your little man