Gotta keep the warrior genes alive, no matter how hard some do-gooder tries to stamp them out. Even Prince Charming had to kick some butt to save the fair maiden.
Talk to your son and reinforce that he's always A (the) Good Guy, only shooting imaginary "Bad Guys" and to never point his gun's muzzle (or his finger) at anything he doesn't want to destroy, that his finger is always loaded (imaginary bullets... tho sometimes he'll have to reload, this isn't TV or the movies ya know
), to keep his trigger off the finger until his sights are on the target (OK, not to go "Pow" until he's got 'em in his sights) and finally to make sure that his (imaginary) bullets don't hurt anyone else behind his targeted victim, er "Bad Guy"...
He'll be good to go.
Oh yeah, tell him to clean his gun once he's done fingerpainting for the day too.
As for the "teacher/watcher"... all I can say is, "Some people's kids"
You could always buy him some toy soldiers or a couple of GI Joes, cause no matter whatcha do, boys are gonna be boys (thank goodness).