I was the kid who got sick in the back seat. As an adult, I typically drive long-distance trips, because I still feel rather unpleasant when I'm a passenger. And I've got a kid who inherited my stomach. Poor boy. Stop and go traffic often makes things worse for motion sickness sufferers, so I wish you a smooth ride, with no traffic to speak of!
I'd go for the motion-sickness meds. You can check with your pediatrician about the exact dose - I know our doc has occasionally given us different dosages than what's on the bottle...
Make sure his stomach is never too empty... or too full. So during your drive, aim for small, light meals, small snacks in between... In my experience, you're more likely to get motion sick on an empty stomach, and a very full one can make its own nausea when on the go. Ick. You should also keep your son hydrated. Don't let him guzzle, but have him take small sips of water at regular intervals. If you let him have soda, ginger ale and even Coke can help. Again, in small sips. Guzzling is NOT a good idea for a motion-sick stomach! I'd also bring a simple, salty snack - something like pretzels or maybe Saltines. This might sound really weird, but sucking the salt off the pretzel (or even eating a few) can help keep the queasiness down. Let the kid have some air while you're on the go - crank down a window on his side of the car. Fresh air on the face can help. I know that's more comfortable in April than in August - if you have a backseat a/c vent in the car, make sure it's pointing at him for your August trip, and open the window when you're going at a good clip - the heat won't feet quite as obnoxious when it's coming in at 65mph. And of course, make sure he gets out and walks around at any rest stops you make. That will both give him fresh air and a chance to move and NOT be stuck in place longer than necessary...
Keep several sealable bags in the car, just in case. And paper towels, perhaps a full towel he can drape over himself if he starts feeling ill, a full change of clothes readily accessible without unpacking the whole trunk, and some cleaner/Clorox wipes.
Good luck