re: 9 year old son. Right handed, left eye dominant.

I am cross-dominant. And lots of people are. What to do depends upon how cross-dominant you are. In my case, I have always either squinted or closed my left eye. Works fine. Both eyes open doesn't work for me. In the worst cases, some have to shoot lefty as a last resort. I would not go straight to that option. Experiment with other less drastic alternatives first.