If your son is mostly interested in bench rest shooting, I would recommend doing both: high/offset rings and a modified stock. A scope on a heavy .223 rifle will not have a problem with the offsets.
I would attach 45 degree accessory rials to the rail on the receiver, then mount the scope to the 45 degree rails using extra high rings. Then I would mod the stock to get a comfortable cheek weld.
The key is getting a comfortable shooting position that doesn't put strain on his neck. If the shooting position is awkward, he probably won't get tight groups.
Cant the rifle if that's what works best for him. When he works out the best shooting position, rotate the scope in the rings so that the reticle is level in that position. Attach an anti-cant indicator to the scope tube (not the rail) so that he can always get back to the same rifle cant.
Zero the rifle normally. Be aware that changing target range will always require a significant windage adjustment. For hunting he should zero differently. Let me know if he is planning to hunt or change target range frequently.
Ok here is the problem....My son has to shoot right handed but has to
