Honestly, it's really a developmental milestone going on - around this age, babies start to realize that, even though they can't see you, you still exist (before then, they're neurological reasoning is that, they can't see or hear you, or a toy, you or it no longer exists and they move on). This new realization also explains the first bout of separation anxiety - he realizes that you aren't there, and it makes him upset. Keeping a steady stream of talking, singing, etc, as well as frequent stops (yes, I know it's a pain in the butt to make a stop halfway to the grocery store, but it WILL get better). Hearing your voice should help him, and you can also get a soft mirror like this one (http://www.walmart.com/catalog/product.do?product_id=4269003) so that he can see the back of your head (although this might only console him for a minute, because he won't be able to understand why you're up there looking the other direction. If you try all that and he's still miserable, you might want to consider switching to a different car seat (I'm assuming you use an infant car seat "bucket" or "capsule"). Convertible car seats like the Cosco Scenera 5 point or the Evenflo Triumph Advance (I can give more recommendations if you go this route) tend to be more comfortable, and give the baby/toddler more of an ability to see out the window and be more comfortable in general. Also, when he's a couple months older, you can adjust the installation of the car seat so that his seat is more upright (you should maintain a 45 degree angle for infants). This also might make him happier.
And a general PSA: babies and toddlers can see a lot more rear-facing than you think, especially in a convertible car seat. The American Academy of Pediatrics recommends you keep your child rear-facing as long as possible, and for very good reason (http://tinyurl.com/2smdts, http://www.msnbc.msn.com/id/9916868/, http://tinyurl.com/gvh7b). There is a great video on YouTube about the view from a rear-facing car seat, and I think it's fantastic (http://www.youtube.com/watch?v=Vj4xEOP7yWk). If you'd like more information about rear-facing, please let me know. Car safety (for everyone) is very important to me.
~H. Ross, NHTSA certified Child Passenger Safety technician.