A good basic gun that he can grow with and find to be a keeper when he is ready for something else. I assume it is a 20, which may be plenty as he grows but if not there are standard size 12's aplenty.
Mossberg 500 and Remington 870 express in 12 gauge are both great starter shotguns. They both can take a lot of abuse and continue running. Both are reasonably priced and both have a lot of aftermarket parts available.