I am an adult and ToT terrified me. My stress nightmares tend to involve out of control elevators so, yeah, the ride is a waking nightmare for me. I rode it once with my daughter because she really wanted to go. I had to apologize to everyone in my car because I swore loudly and repeatedly during the ride. I had to put my hand over my mouth to stop. Interestingly, now that I've been through it once I would be willing to do it again.
When my daughter was around the OP's son's age, I talked her into Splash Mountain and she absolutely hated it. She is 16 now and she still won't go on it even though she will ride everything else at WDW. No matter how many times I tell her it's nowhere near as scary as ToT, Space Mountain or Expedition Everest, she still won't consider it. I regret pushing her when she was too young.