I'd start by giving him the dominant eye test - Of course if he closes one eye, then this doesn't matter - This is the first thing I do with new or inexperienced shooters - I've come across a few that were right handed, with a dominant left eye; two choices; close your left eye when you shoot, or shoot left handed.
Next, I would get someone to throw clays, first start shooting at targets straight away (usually they shoot high, but you can tell and correct him), then move him to shoot at crossers - while you stand behind your nephew. Have him hold the gun as he would hunting, and mount the gun after the target is called. You can see where he is shooting, and help him adjust. You can build a lot of confidence in a new shooter, as they start busting clays. (I really enjoy working with & helping new/inexperienced shooters)
Having him mount the gun after calling the target helps him learn proper mounting technique and target acquisition.
Good luck and sure hope the two of you are celebrating this year!