I would think that the Avistar would be better for him to start on, but if he gets on a sim, and with a buddy box, he will probably be ready for the SSE soon after.

Enjoy the project with your son, and let him do as much of the work as he is willing to. My kids tell me I tend to take over and do things myself. I built a SPAD trainer with my 9 year old daughter this winter, and am building another one with my 10 year old daughter right now. My 12 year old son wants to learn to fly on my Alpha, he is not interested in building.

Good luck.