I'd recommend a Beretta RL 391 with 1 oz or 7/8 oz loads. Speed is relative, keep it under 1200 fps, he'll be fine. Make sure, if you get an auto, you also either get a shell catcher or deflector.
The more experienced (with kids) shooters will probably recommend something better -- this is my suggestion.
The more experienced (with kids) shooters will probably recommend something better -- this is my suggestion.