It could be the fact that he can't see you - my DD LOVES riding in the car (and now that she can talk, is constantly begging to go for drives in the car) and when she was a newborn she would mostly sleep in the car (the motion sent her to sleep) but when she wasn't tired she would scream because she couldn't see me - as soon as she put her capsule facing forward where she could look around at the road and DH and I sitting in the front, we NEVER ever heard another peep out of her in the car (except for happy noise lol).

I know it goes against the gentle parenting ideal, but sometimes (and this situation is one of them) you just have to let them go... I know how harsh that sounds but when you gotta be somewhere, you gotta be somewhere and you can't pull over every five minutes to comfort them - they just have to learn to deal with it... Unless you want to start leaving for your destination two hours in advance so you have all that extra time to be in and out of traffic, comforting him!!

Things should improve when he's big enough to face forward - we put DD's carseat forward much earlier than we should have but she was a really big baby (ie the weight and height requirements were okay, she was just younger than the recommended age) and once she was facing forward and could see that mummy and daddy hadn't left her, she was fine, and we can now do long-distance drives with zero problems.