My mum had the exact same problem with my half brother when he was a bit younger
What we did is we told him the car wouldn't go if his seatbelt wasn't done up.
Whenever he undid it we'd pull over and stop the car.
luckily he didn't use this against us and take the seatbelt off when he didn't go anywhere, he always wanted the car to go .
I think it may of also helped that we named the car and because young kids think all toys and objects have feelings he didn't want to dissapoint the car and make it go to sleep
worth a try, but it may backfire if he doesn't want to go anywhere anyway