Eye dominance- use the tech described above and/or
holding his thumb up at arms length and covering an object a few feet away with BOTH EYES OPEN-THEN alternate looking with just one eye and one closed to see what is happening with thumb (stay covering the object or shifting-if it stays put , that is the dominate eye) and/or
have the student point a obviously inert orange training gun at you that has had the rear sight opened up so it can be used to demonstrate equal height/equal light and see which eye seems to be favored.
These techniques plus factoring in his dominate side should reveal his dominant eye.
Pistol shooting with cross eye dominance issues is easily surmountable relative to the dominant hand side, long guns, not so much.