It’s a pain, but even recognising that wanting the car seat done up is your desire not his is useful.
There’s nothing in it for him, having the seat done up. He isn’t motivated by being safe or not dying. When you realise that it’s easier to find something that does motivate him (not earning stickers, bribery etc- at least not for us).
Having stood at the side of the road for ages with a DC while they refused to get back in the car, I feel your pain. On that occasion the thing that got her back in was being able to ring her social worker when she got home, to dob me in. By then she no longer wanted to, but in the moment the opportunity to complain about my awfulness to someone was irresistible.
You could try a game/challenge. “If you can keep your car seat done up nicely all the way to school, I’ll hop all the way to your classroom!”. And when you initially put him in it, make a deal out of checking it’s on right, comfortable, not pinching, ‘tight enough to keep you safe, but still comfortable’ etc.
Jokes like that worked well with mine. “Oh my! Have I done it right? You’ve gone a bit quiet! Can you still breathe?! Phew😅! Thank goodness!”.
Or a job like, I need to find a cake shop so we can stop on the way home to buy us each a little cake. While I concentrate on driving the car, can you check out the windows and tell me if you see a cake shop?
Distract, delay and engage. Tiring, but actually quite good fun.