I'm in agreement with those above who recommend a shooting lesson or a few first. Assuming he's about average in orthopedic construction, an about average gun should be somewhat suitable for at least a session or two. But leave that to the professional.
I've watched some shooters who looked very average, make mounting a gun into a contortionist act. I've tried to intervene and correct the problem and couldn't seem to help. After several years of watching one of these shooters mount a gun ackwardly, I started to see a shift to a more natural shooting style. A few lessons likely would have prevented this early on.
