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At that age, when I sat in the back with LO it would help a lot. Have you tried that? Otherwise I would try at bedtime and see if he will fall asleep.

My LO was a terrible night time sleeper and hated the car. But shockingly did really well on road trips! Definitely leave at bedtime and sit in the back! Bring bottles if he takes them. Don't stop unless he wakes up or for gas.

We have done a few 4hr drives and I sit in the back with toys and snacks and bottles of milk. I hope it goes well.

@Leah: We just did a long road trip with our 11 month old. It was supposed to be a 10 hour drive and we made it in 11 which felt like a major victory. Here are a few things that helped:
-Sitting in the front during nap and nighttime sleep times. My daughter won't sleep if someone is in the back with her, so when it was around her nap or nighttime sleep time, I sat in the front. She's not great at sleeping in the car but she did it eventually. We gave her a blankie or stuffed animal to snuggle while she slept. We listened to an audiobook and adjusted the car speakers so they only played in the front during those times.
-Sitting in the back during awake times. I hardly ever do this at home but on a long road trip, it really made a difference. We switched up me and my husband back there for some added variety.
-A big bag of toys. I brought a big bag of little toys and books. I got two or three out at the beginning of the trip, and then I rotated them in and out as she got bored.
-Stopping efficiently. We brought a ton of water and snacks for us. I even packed us dinner. That way, we could stop every 2-3 hours and all at once do a gas fill up, bathroom breaks, diaper change, bottle, give the baby a little time to hang out outside her car seat, and then hit the road for another solid chunk.
-Put him in nighttime diapers to help avoid diaper rash.

Good luck!