6 years old...
I say get him a kids book on "How airplanes fly", and a bunch of gliders, maybe some with a rubberband motor. To this day, I still have fun playing with these, trying to get the longest flight I can. If he demonstrates that he wants to learn more about aeronautics, then go on to more advanced forms of modeling. I've seen one kid and my impression of him is if he's not doing it, he's not interested. He's off wandering around the field messin with the cattails and not paying any attention to the flying. I think this kid just thinks RC is just toys. Certainly I have fun with my RC planes, but I also feel there's more to it. Its a hobby and a sport, a way to be creative, as well as being truly incredible. Minature aircraft.
Personally, I wouldn't just give him a RC airplane. Time on a buddy box, to wet his appetite, you bet. Find out if he really has an interest. If he's truly interested, he'll study any aeronautical item he can get his hands on. Small steps in the beginning, is the way to go.
Good flying, Tom