Shooting a bow and shotgun being right handed and left eye dominant, teach him to close left eye. I learned later in life.
Scoped rifle the same it just becomes easier with the scope.

Turkey hunting put a red dot on the gun and let him shoot. Dots on ......dot kills Shooting dove or ducks wear a patch or learn top close your left eye. Works for me.
Looking down the barrel at a gobbling strutting tom will cause all kinds of probelms for a kid with RH left eye dominance. Guess how I know.. LOL

Good luck