Hi Renee! How fun that you get to introduce your husband and son to Walt Disney World!
Both Big Thunder Mountain Railroad and Test Track have height requirements of 40 inches. If your son is at least that tall, he'll be allowed to ride those attractions, but of course, I understand your concern as to whether they'd be appropriate for him. Since this trip will be his first visit to Walt Disney World, you'll probably want to start him out on some of the slower rides, such as those in Fantasyland. After that, let him take a peek at the kiddie coaster, Goofy's Barnstormer, and if he thinks it looks fun, take a ride.(It's a very short ride, so if he hates it, it will be over really fast.) If he can handle the speed and small dips of that ride, I don't think he'd have a problem with Big Thunder Mountain and Test Track. I have to tell you that I'm actually a big chicken and avoid many of the thrill rides at Disney,but I love Big Thunder Mountain and Test Track. Neither of them have big drops or anything scary in my opinion; they're just fast and fun. If you're son enjoys going fast, I think he'd love them!
Hope you have a wonderful first family trip!