Since he is still young and figuring things out, you can do what others have stated using the patch or closing one eye, and trying left handed to see which is more comfy for him. My dad is rt hand shooter and left eye dominant and is an excellent shooter, closing his left eye.

My buddy is left eye dominant but found it hard to find left handed guns. You can train yourself to be right eye dominant. Just cover your left eye when you shoot. And practice alot.

"Hey ya'll, watch this"