There are a lot of things to consider first, before we can help you out.
1: What type of hunting - lots of walking , jump shooting, or layouts?
2: Has he shotgun experience yet, and how well does he hand a big gun - my son was 12 and shooting 200 birds a weekend , but I have seen kids older that could not?
3: Does he plan on hunting upland with the same gun?
4: How many different guns has he shot, and does he show a preference to an action type?
5: what is the budget we have to work with?
short answer is "Remington 870 3" 12 or 20 ", but that is a VERY big generalization!
I'm sure once the crowd here finds out his preferences , and his abilities, there will be other answers!
Cat