Have him shoot rifles left-handed, if possible. It'll only be awkward until he starts getting excitingly small groups
Another choice is to mask his left eye, and have him shoot right-handed.
Use frosted Scotch tape or smear some unscented Chapstick on the left lens of his shooting glasses (you want light to come in, but no "picture").
This will force him to use his non-dominant eye, and he'll shoot pretty good once he figures it out.
Of course closing the dominant eye works, too, but not as well.