Sim and Buddy Box are the easiest jump into RC. I also have a 6yr old but he has been exposed to it since before he could talk. I started him on a sim and now he flys on the box just cause he hates crashing. We also have a lite stick we fly in the street. Something no one has mentioned is model rockets, they are cheap, exciting, and fun to build. The latter may be the best way to spend some time with the little guy. My son may like building an flying rockets more than the planes. The main point from my experience is spend time togeather. Have an area where the two of you can do whatever you like, we like to go in the shop and cut foam to whatever comes to his mind and then put some paint on it.