If it is practical, I would start off with a bolt action .22LR. You can teach him so much more about fundamentals and consistency with that than with a shotgun. The skills he learns with a rifle will set a good foundation for shooting a shotgun safely.
A great drill with a single shot 22 is to place bullets in balloons, blow them up and place them in a field or fairly easy wooded terrain. Start the child off with two bullets, they then have to find and shoot the balloons to recover more ammo. Great drill to improve their situational awareness in a calm, fun environment. How many hunters do you know that will shoot a deer and then have no idea where to even begin tracking it from? This drill will help eliminate that at an early age. You just have to be tough enough to not give them extra bullets when they run out... I recommend having a plan for that situation, it's not easy to tell them they are done, you can always have them do something else to earn the bullets (push-ups, run, recite the Pledge of Allegiance etc..).
Also, when you do put them on a shotgun for the first time, use the absolute lightest loads you can find. They are already going to be nervous, don't add to it by hammering their shoulder. Trust me, I learned this the hard way. I was all excited to take my son turkey hunting and bought him 3 inch magnums for his 20 gauge... He had never shot it prior to that. He's shot plenty of rifles, but never a shotgun. So, we do a little stalk up to a target and I have him crouch down to shoot it, just like the real thing, he shoots, the shotgun fly's up and out of his hands and he fly's back and falls on his butt. Oops... To make matters worse, I couldn't help it and I laughed so hard I almost cried. Really bad job on my part and he still won't touch that shotgun. Maybe next year...
Oh, and I bought the Rossi combo as well. Mine has the 22LR, 20G and a .243 barrel. My Wife took her first deer with the .243! It's a cool combo and will get the job done. If you are looking to make him a pro wing shooter, I would look elsewhere. The shotgun is not "right" in a lot ways, but it's a decent way to start them off inexpensively.
A BB gun is also a great start. Just don't forget to drill in the basics and wear eye-pro!