Careful there, I'd take the other side of that bet. I've know a few folks who are very motion-sickness prone (inner ear issues) and they had to take medication in high school just to be able to ride in the car, so I wouldn't jump to the conclusion that it's anyone's driving that is the culprit. I mean you said 100% so that doesn't really leave you any room wiggle room.
Anyways, perhaps having your son sit upfront and focus on the road in front and not look at any screens, books, etc would help. after Ginger candies or Dramamine help too. I'm prone to seasickness, airsickness, and had a terrible bout of vertigo in my mid 30s...but I'm still have to sail, ride in a plane, and ride rollercoasters after learning a few management techniques.