I'm so glad you insist on your son riding in a car seat. It's one of the best things you can do to keep him safe. You're also right not to want to reward his outbursts with treats or snacks since he'll learn to have a tantrum even more often if he gets a reward for it!
Part of your goal is to get him to feel more comfortable in a car seat. One way you can do that is to bring the car seat inside your home for a few days. Just leave it on the floor. Strap him in it for a few minutes every few hours while the two of you do various things, such as feeding him a bottle or a small snack, or playing with some toys. Get him used to the feeling of sitting in the seat. If he becomes upset, try to distract him. Wait until he's calmed down before you let him out of the seat.
After a day or two, put the seat back in the car. Place your son in it, but don't drive anywhere. Just talk to him and play with some toys together. He should be much calmer. Again, if he becomes upset, don't take him out until he's calmed down.
This type of practice should make him more comfortable riding in his seat during car trips.