I'm going to ask you to please do not do what my father did to me when I started to hunt. He bought me a 22/410 O/U Savage. It was probably one of the worst guns you can give a kid to start with. The .410 is nearly worthless for a young guy starting out. As mentioned before, it's best used in the hands of experienced shooters, not by someone just beginning. I remember taking at least three shots to bring down a squirrel in a big oak tree. If I'd been using a 20 ga., only one shot would have done it. It only throws a minimum amount of shot, and when you are a young, jumpy kid, that's a big handicap.
I would buy him a 20 ga. pump that he can handle. I personally don't like any of the single shots out there on the market today for youth shooters. Most of them produce way too much recoil for a little guy. JMO....