Welcome to the Disney Parks Moms Panel, Shanna! My 11 year-old daughter is very similar to your son in that she isn't a roller coaster fan either. She loves so many of the fun and even fast attractions at Walt Disney World, but roller coasters are an absolute no for her.
Soarin' is one of her favorite rides. It does not resemble a roller coaster-like ride in any way, so she adores it. Soarin' is a virtual ride, in that you'll be watching the huge IMAX screen that makes you feel like you're actually flying over these amazing sights throughout the world. Guests are on row of seats that are lifted up in the air to simulate the flying. If she's ever nervous, she just closes her eyes and feels the minimal movement, but without the visual in front, it's not a big deal. However, she adores Soarin' and I am sure your son will too.
Now,
Test Track is another story. It is not a roller coaster in my book, but in hers, it might as well be. It's fast at certain points, and on a track, so she does not enjoy it at all. She has been on it a few times, as she gives it a try each trip thinking she might enjoy it (which she doesn't do at all with real roller coasters), but while the rest of our family wants to ride again and again, she's out for those rides each time. So, taking that in mind, if you had to choose between Soarin' and Test Track for your son, go for Soarin' 100%. Perhaps if you would like to try Test Track, you could pull up some videos of it on YouTube to show your son and see how he feels about it. My daughter "tests" different attractions this way before we leave for trips to decide if she's up for that ride or not.
I hope that helps as you plan your Disney vacation, Shanna!
Marcy