DS was totally like this. We found that loud heavy music really helped, so I think he had a bit of motion sickness, and thats why he screamed. The girls have never had a problem with the car, so I think its what it was.

We used to play Ministry really loud and he would be much better - I think it was the beat that was soothing against his seat cos we had a big speaker in the boot, so he could feel the boom boom boom against him.