Do a close range shot on something like a ripe Tomato or similar, to show the damage that it can do if neglected.dont put it in away that will make him run a mile. but drill it into him that a liftd mask can lead to been blind and the worst case is death ect ect
As to "pump or semi", well you could argue either side, but I would say pump first would be a good bet.
My children (the eldest two) started shooting real guns with me at about 6 years old, they know the dangers, the routines and the full on safety, with added GORE.
You can't sugar coat some things, it's more dangerous than not telling them.
She's 10 now!