Hm. Hasn't he been able to survive great injuries in the human world as well?
But those might have been humorous exaggerations. For the now, I'll accept the idea that the extra superior ability to survive (even gunshots to the head) might be some kind of reversal of the between-world thing. Just like charger girls become more ephemeral, he becomes more solid.
(Although there's clearly thing they don't know, and he might well be related by blood with someone in or from the Charger world..)