Hi Anne-Marie ~
I am sorry to hear about your son's motion sickness issues. I actually asked a Cast Member at Star Tours on my last trip to Hollywood Studios about non-moving seats. Unfortunately, they do not offer them at Star Tours at this time.
There are a couple of other simulator type rides in the parks. Mission: Space at Epcot offers a non-spinning version that is less intense but its still a simulator type ride so that may affect your son as well. Soarin' also at Epcot is projected in front of an IMAX screen and while the ride itself doesn't move a lot, it gives the image that you're moving fast across the land in a hang glider.
That is a really tough age to come to Disney and not be able to do a lot of the bigger rides since your son may feel he's outgrown some of the slower moving rides. Perhaps you could book a
special experience for him that he would enjoy. Dolphins in Depth at Epcot has a minimum age of 13 and provides an excellent opportunity for interacting with the dolphins. You could also look at Epcot Seas Aqua Tour. The Wild Africa Trek at Animal Kingdom is amazing. Just be cautioned you are on a safari truck for part of the time and there are two high swinging rope bridges that you cross. I'm not sure if either would bother your son or not.
If you have any questions about any other specific attractions, I'm happy to assist you.
Wishing you minnie magical memories :-) Allison V.