He'll remember his Uncle for that one moment, and never forget. The thing you can never tell him, is that you probably got a bigger kick than that shotgun gave your Nephew.
Great job! I love being there when a kid gets their first. My son just completed his Hunter's Safety course, and now I get to really enjoy the hunt through the eye's of my boy. I was thinking about setting him up with a 3 and 1/2 inch out of the ten gauge. Or do you think that will be too light of a load for a twelve year old?