Technically he wasn't decapitated. Decapitation is when your head is totally removed from your body. There is a name for what happened to him although I can't remember it right now. It's when everything is severed EXCEPT the spinal cord. It's obviously what happened to this kid, because there's no way to reattach a spinal cord.

That said, it makes it no less fucking amazing that he's up and walking around, much less alive. Makes me wonder what he'll be when he grows up - he's obviously got something important to do.