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

Casual experience here, fwiw ... my Son is left handed, left footed (that's important when evaluating) but, he's right eye dominant.

I started him shooting at the age of 4. By 6 he was winning his age class and above. By 18 he was competing nationally.

He's older now, about to marry but .... I raised him to shoot right handed rifle for two reasons.

1 - he is right eye dominant.

2 - he has more rifle options as a right handed rifle shooter.

When he was shooting 3-gun, he still shot pistol left handed.

Nothing more frustrating, as an instructor, than having crossover students (opposite dominant eye and hand) come through my class trying to shoot right handed while left eye dominant .... especially when it is a woman. They cannot grasp both eyes open sight acquisition.

I mention this because it matters a lot more with pistol than rifle so ... why not train them early.