The pup will sleep most of the time. Ideally, you have a passenger to reassure it; after all, it has only recently been separated from its dam and siblings. It will probably want to urinate when it wakes, so be prepared. Unless travelling by train, I don't bother with a carrier or crate. Stopping is not without its risks. Parvovirus is fatal to puppies, yet it may be carried by adult dogs without ill-effects. It is transmitted through the faeces of a dog and may be caught from anything which has come into contact with the faeces. I avoid stopping, keeping it to the minimum if at all. It is easier to let it relieve itself on suitable paper in the car, but I appreciate that , in some vehicles, that may not be easily accommodated.
In my time, I have brought pups from Newcastle, Bradford, and mid-Scotland to Cambridgeshire and have never found any distressed or stressed by the experience. Young pups are easier in that way than older ones and seem content in the journey.