There are degrees of eye dominance and until you have and idea re how much, it could be 1 or 2. Until then 3 is the correct answer.
Some have central vision and neither eye is strongly dominant, in which case you may have him shoot left handed and the problem will still exist.
Changing a strongly dominant eye to become the non-dominant eye is a task that few will accomplish.
With a lot of shooting some have learned to shoot from the non-dominant shoulder with a shotgun. If they start young and shoot a whole bunch of a lot, the brain may be able to figure the correct subconscious pictures. Most never shoot enough.
Studies have shown that in hitting a baseball there are advantages to being cross dominant. I.E. when batting right handed with a dominant left eye, the left eye picks up the ball and locks onto the ball easier than the right eye of a right hand/right eye hitter. It has to do with head positioning from the start and maintaining head position until completion of the swing.
For some all it takes is a slight squint of the non-dominate side eye at the moment of the shot to verify they are looking at the target with the right eye.
Perhaps may be the best choice of all. Some highly accomplished shotgunners without eye dominance issues still do the same.
