babe in arms, excuse typos
we just got back from a trip to DH's family - about 4hr by car!
I have a spit-upper (and my dd1 is occasionally car sick) so my #1 tip is tp be overprepared for messes. Have everything you need to clean up a big puke or diaper blowout in one bag, easily accessable so you don't have to dig in the trunk on the side of the highway . Wipes, papertowel, cloth diapers, a wetbag or plastic bag, several complete changes of clothing, a clean blanket (or two), a bottle of water and some wascloths, all at the ready.
You might want to lay a light flannel recieving blanket under him to keep the seat clean.
Related - make sure your backseat isn't too hot. Ours gets warmer than the front and that makes them more likely to be sick. light clothes plus a blanket instead of heavy, warm clothing
Claire slept the whole way, both ways. I did take a soother just in case, even though she doesn't normally use one. Better than hours of screaming in case of gas etc
If he's awake for parts of the trip, you might want to sit in back for a little while so he isn't too bored. and/or bring some toys to hang for him to look at
Packing for 2 nights.....again, she spits up so I pack lots.....I would pack as many outfits as you can - they don't tke up much room.
Are you taking cloth or disposibles? I would take a bag of disposables or most of my cloth - you can keep most of them in the car and just take in a day's worth or whatever at a time
Take plenty of blankets/recieving blankets, burp cloths etc, whatever you normally use
I'll ptobaby think of more, later :)