Go to
www.carseat.org (safetybeltsafe.org) and print out the 5 step test. Have him do it
himself.
Another demo to use is to put a tube of toothpaste inside an empty paper towel/tp roll. The toothpaste is the human and the cardboard roll is the seat belt when it rides up off the hips/legs. Bend the toothpaste tube to show the internal injuries.
You could also show him in the car - give the shoulder harness a good jerk (ala CRASH - hitting the brakes) with the seat belt positioned correctly and with it riding up onto the abdomen.
From there, it's simply, "My car, my rules," or "Because I'm the adult." Or as one of my resource kids once
defended me to his peer, "She's not mean, she's just . . .picky!"
Most kids are fine with that. Especially since your kids clearly are in the same "boat."
FWIW: My nephews (7 and 9) have never complained (at least to ME
) when I adjust their seats, boosters, seat belts or head restraints or in several cases put them "back" into a booster in my car or theirs. They may ask "why?" but that's been covered.
Happy taxi-ing.