Get a little travel crate (some come with a 'flap' to run the seat belt though) and belt it securely into the car. I'd put something like shelf liner in the crate and then a towel or something on top so the dog doesn't slide around (that's scary!)
Give it a lovey toy and stop every one - two hours and let it walk around and sniff and pee.
We traveled from VA to PA with a 3 month old puppy (he died last April at 12.5 years!) who it turned out did NOT like car rides (but he went from VA to Pa-and back and VA to FLA-and back and then VA to WA state and then WA to OR and then OR to TX!!!) but did ok actually.
That first trip we made a lot of false stops when he whimpered or whined 'cause we were afraid he had to go but their instincts DO tell them not to go in such a small space, and he had not an accident any of those trips. He was barely a year old when we drove cross country from VA to WA too!
Good Luck,
C.
ps perhaps the current 'owners' could introduce him to a crate before you get him, maybe even sell the one they use with him to you to take back. It might make him more comfortable.
