It's best not to start in the car, but rather in the animal's usual environment. At home, the dog can learn what the new seat actually is without fear or stress. He can sniff it, use it as a toy and even lie down in it.
It is important not to put pressure on the dog. If he is not interested in the seat, you should not push him in, but gently attract the animal's attention: a few treats or a toy will help.
If the dog has accepted the car seat, it is quite possible that he will soon take it for granted like his basket. Then you can start using it in the vehicle.