I agree with checking the eye relief and focus first.

Get a sight picture yourself, and measure the maximum distance your eye can be from the ocular lens without losing field of view. Make sure he is at least that close, and use a caliber with negligible recoil, like a 22 so he isn't afraid of snuggling up close.

The focus can be different for him than it is for you, so let him adjust it for himself. Remember to takes breaks after each adjustment so his eye relaxes and he can tell if it made the picture better when he looks with a fresh eye.

You could also consider a red dot with no magnification.
It's very easy to shoot those with both eyes open, and then he'll learn topay attention to his right eye since that is the only one that will be showing him the dot.