Answer
Jack:
Definitely a crate, and a largish one for his small size. He will have accidents and the crate will contain them.
Get some of those wee-wee pads, and line the crate with them. Change them as necessary, of course.
Be sure to use the seat belt to secure the crate to the back seat in the event of an accident or needing to slam on the brakes.
Offer a bit of food and water every couple of hours. Do not be letting him out at rest stops, as you have no idea what sort of germs may be there - especially parvo.
Jaroslav:
Of course you need some food, water and let the dog minimally every 4 hour get out for 15 - 30 minutes to piss and get away.