When dd was that young, she was the exact same way! It was horrible! I decided to just stay home until dh could come to the store with me. This way, I could sit in back with her. She still cried but at least I was right next to her. At some point (sometime before she was a month old) we got a mirror that hangs on the back seat. That helped! She could see herself and I could see her in the rear view mirror (everytime she stopped crying, I'd think she stopped breathing) Also, we got a toy that clips onto the straps of seat and she would play with that. When she was little little, she'd just hit it and make it crinkle, but as she got bigger, she started really loving it. She still loves it now at 11 months old.
And, with time, he should out grow it. DD is 11 months old and only cries in the car if she's really tired. She stopped the constant car crying at around 5 months.
Hang in there and congratulations on the birht of your son!