Six years old , hmmmm, thats pretty young for RC. But not too young for models. There are some great rubber powered models or even the balsa and tissue gliders. Heck , there are great simple balsa and rubber models at the Dime store, ( or what ever they call them now). Remember, the fascination is with airplanes, not necessarily RC airplanes. Heck , flying a free flight glider is a thrill, even at my age, (43).
I started with plastic models, and at the age of 6. You might consider them. I believe he would be very thrilled with something much simpler than RC.

Oh, I'm not suggesting that you completely omit RC. Sure get a buddy cord and let him fly with you from time to time. And by all means let him play on the sim, if you have one. But too much too fast could jade him. What will be left to thrill him later, tight ropes and bank robberies ?

There are some fantastic paper airplane books. They have the airplanes printed in them in color. You just pop them out and tape them together. And they fly ! There are toys to consider, like those little helicopters, Verdi-birds. They are a hoot. And let us not forget the COX control line airplanes !!! Absolutely mandatory. The gateway drug of any true aviator. RC is not step number one when growing up into a life of aviation. At least it didn't used to be. There are many wonderful and enchanting steps that precede the digital wizardry of RC.