Personally, I'd recommend a pellet gun in .177 caliber because its inherent accuracy will make it more enjoyable for a longer period of time, perhaps even after he graduates to a firearm.
I agree with this (though I learned to shoot on the aforementioned Crosman Pumpmaster
). Be aware of whether the airgun you buy is powered by a pump handle, or by CO2 cartridges. If it's CO2 powered, it'll be another thing you need to buy (not that they're super expensive) in order for the boy to shoot. I suggest going the pump route, and there are several great options that are inexpensive - many of which you can find at a WalMart. I strongly suggest an air rifle, rather than a pistol, especially if he knows he wants to move onto hunting eventually.
One thing I want to emphasize is eye safety. Even though it may seem trivial, pick him up a pair of basic shooting glasses (there are quality pairs for around 15 dollars), and
make sure he wears them. I had many a BB bounce back and come close to hitting me in the eyes when I was growing up. Like MedWheeler said, it's much more unlikely that a pellet (as opposed to a BB) will ricochet, but it's definitely possible.
I think something like
this Crosman 2100 Classic would be a great choice. It's pump-action, moderate power, able to fire BBs or .177 pellets, and a great price.
If you decide you'd rather go the CO2 route, rather than pump-action,
this Crosman 1077 Repeater is outstanding. My parents bought me this exact airgun when I turned 12. Can't believe they still make the thing, to be honest. It was great though.
Welcome to The High Road. Good luck, have fun, and be sure you supervise him. If nothing else, the safety habits he develops right now could be the difference between a life saved when he moves on to more powerful rifles and handguns - whether his life or another person's.