Always stop after 1 to 1.5 hours on a longer car trip, leash the dog securely. If he does his business outside the car praise him very well.
Do not let the puppy feed on any short stop, offer only a little bit to drink. Don't worry, the puppy can survive half a day without food - the next meal can take place at home as a first strong bonding step.
Once you are home, drop a sign to us, that you arrived well home with the puppy.