The standard advice is to wear clear safety or shooting glasses and cover the dominant eye lens with a piece of opaque plastic (e.g. from a 2 litre milk bottle). This allows the shooter to keep both eyes open, but only process information from the non-dominant eye. That is what they taught me to do on a junior coaching instructor course, but I've never had first hand experience myself as I'm right eyed.

Edit: but if you mean he is blind in one eye, that is different! I think shooting left handed is going to be easier than any kind or 'reach over' stock or scope mount..