Henrika
Wait I'm confused, why can't you bring the cat in the car? How long is the road trip?
The road trip is for a week.
But anyway, I agree- just take the cat in the car. Get a nice crate, a litterbox that can fit inside of it, food and water, and you're good to go. Before you go on your trip, I'd make sure to acclimatize him to being in the crate for extended periods of time and being in a car so it'll be easier for him to adjust.