We just got back from a looong, multi-state road trip with our 14 month old. Where is DS's carseat? Is he facing forward? Our son was in the middle, facing forward (he's HUGE) and that seemed to help distract him (watching out the windshield).
So did a couple new toys (wooden truck and frog) that were kept in the car, bubbles (INVALUABLE! I would blow them whenever he got fidgety :-)) and most of all, lots of snacks. I normally hate those lil fruit smoothie pouches for all the packaging, but they were really convenient - also, some fresh fruit, cheese sticks in a cooler, crackers, teething biscuits, etc. And, of course, a sippy cup filled with water (which sometimes was used to amuse himself by dripping water out onto the seat - but hey, it's water, and kids dry!).
We would also put on music for "dance parties" or sing songs. We stopped fairly frequently (every couple hours) at rest areas where he could be outside, run around, play with sticks, use the potty, etc. We tried not to drive more than 4 hours a day, but broke that rule twice with 8+ hour stretches (one night, one day). He still did great.
Is your DS nursing? If mine got sleepy or anxious, nursing him in the car for short stints really helped keep him calm (not the safest option for you, but effective!). We've taken him on the road throughout infancy, and I find car rides go much smoother if I just commit to riding in the back from the get go. Then he knows where I'll be, I can nurse before it's a meltdown, etc.
And definitely capitalize on naptime/bedtime whenever possible! (nursing really helped him stay asleep if he started to wake up). Good luck and safe travels! :-)