I think Qarin's point is "if" you might be able to reason with him a bit. Since he doesn't think the harness "looks cool" maybe you can say, well, if you sit still the whole ride in the seatbelt, then you don't need the harness. If you move out of place (I would might say "too much" because as an adult, I do move out of position from time to time) then you need the harness again. And then maybe take extra time with each trip, start with seatbelt, if he doesn't sit, then harness. Maybe, he will start to sit longer and longer without the harness. Just a thought.