Lucky you were using CF-W A bolt, and he was old enough to handle a full auto, would've been a pretty bad experience if he was smaller and it was a standard bolt. Remember when that nine year old girl accidentally shot that range instructor in Arizona with a FA? Never really felt comfortable with new shooters handling a FA, especially if they're not even in highschool yet. Runaway or not, I'd be pretty nervous if a kid was shooting a full auto if they also weren't really much familiar with them.
Your grandson certainly handled it well though, I'm assuming he's had experience with machine guns prior to that accident, though he probably should've kept an eye on to where it was pointed towards the end. Still though, handled it better than some people who experience runaways
So was it an ammo issue? or is there a part inside that needs replacing?