You can't go wrong with
Robert. I have known a few who were always called
Robert, so I don't think the nn is automatic, and anyway, once a
Robert has grown up he may insist on any nn he wants.
Bob is definitely not a nn you see on young boys or even young men. You don't even see
Bobby as much anymore. You do see a lot of little Robbies and those tend to grow into Robs.