When I had a dedicated NV on my FX Verminator, because the mount was so high the easiest way to shoot was to roll my head over the stock & shoot it left eye.
So if he needs to shoot left eye I'd say fit the highest mounts you can find & try to roll his head over the stock, cutting down the stock would also help.

I know in the Biathlon world cup one shooter uses a dog leg stock to shoot left eye-right handed after she damaged her right eye.

I know Gehmann do an offset mount system for disabled shooters.
https://disabledshooting.org.uk/more...osite-eye.html