You would have a relatively small time in which to decide whether or not to ride.
If you have not been on the ride yourself , I would suggest asking if you can have one member of your party preview the ride before trying to get on with your son. Rider Swap is common on Soarin' because of the height requirement and nature of the ride. It might be best to fo a regular child swap first so each of you can ride. Then discuss your experience to make a decision about whether even would be possible for him to go.
There is also a post on the EPCOT FAQ thread near the top of this board that shows and describes the things about the ride.
It would also be helpful to know what about a theater makes your son uncomfortable and how he shows his discomfort. Once the show starts, it is not possible to leave, so if he is used to leaving when he is uncomfortable, you need to think about how that will affect him.
The ride is actually a movie which is being shown when you are not in traditional theater seats, but special ride seats. The movie screen is in front of you, curves a little bit to the side and also below and above you so it isn't quite the same feeling as a movie theater where the movie shows in a box in front of you. On Soarin', the whole movie will be in your field of view unless you purposely look a little over to the side, below or top.
Because there is a screen, your son will see a movie screen in front of him if you are in the first row. The other rows can see it as the people walk in, but once seated the view of the screen is blocked by the row in front of you.
The seats themselves are more like lawn chairs with very high backs than regular theater seats. You can't see the people in seats behind or in front of you because of the High backs and all the machinery.
There is about a 6 to 8 foot aisle in front of each row of seats so being in any seat, you can still get out until they are done with the seat belt check. Once your seatbelts are fastened, they lock and the CM would need to release them or you can't get out. Once they have given the all clear, the flight is taking off and can only be stopped in an emergency, with an emergency stop and evacuation.
The ride starts in total darkness, during which, the rows of seats are pulled forward and up.
Row 1 will end up suspended about 35-40 feet above ground. Row 2 is about 25-30 feet and row 3 is about 15-20 feet. When seated in rows 2 and 3, you can see the feet of the guests in rows above you.
The show simulates a ride on a hang glider, so your feet hang free and the rows of seats tilt and sway according to what is happening on screen. You can find videos of the movie on
YouTube and get an idea of when the more noticeable seat movements might occur, but it will be moving a bit all the time.
The movie also includes fireworks at the end and scenes over mountains and water. Scents are pumped in as you go over certain areas ( pine scent by pine forest, orange scent over orange grove, etc.) in case your child is scent sensitive.
After the fireworks scene, there is a short period of total darkness again as the rows come back and down into their original position.
Canada and China have 360 films where the screen is around you on all sides from waist height to about 20 feet high up the wall. These are movies where you stand. The way the movies were shot gives a feeling of motion, even though nothing moves and you are standing still on the ground. Going on one of these immersive movies might be a good way to see how he reacts to a movie that is a little more sensory than the movies he may have been in before. You can easily leave either of those movies if you want just by walking out the doors opposite from where you came in.
I was just about to post this! 
