This guy is often the owner of the car, because no one else but him is ever allowed to drive his sweet Toyota. Typically this guy is going to be more excited about the destination than the actual road trip because that means that everyone will be out of his car, and in a place that is not his car. He won't be able to participate in the trip because he is to busy driving his car and making sure that his car gets to the destination safe and sound. It's best just to leave him alone and let him do his thing.
This guy probably imagines that he is sitting in a car seat to help him sleep at night because he can't sit in one for 5 minutes without falling asleep. And not only does he sleep the entire trip he also sleeps on the ENTIRE back seat. Either his heads tilts onto your shoulder making you shift and ruin the whole ass to seat ratio, or his feet will magically lift themselves up on your lap while you're sleeping. None of you wants to wake him up, because when he is denied his sleep he gets incredibly cranky and irritated, so you will have to sit in various uncomfortable positions for the rest of the 15 hour trip. On the other hand, if you bring a black marker, he can and will unwillingly make up for it.
This guy could never bring himself to pee in a bottle, but has no problem forcing you to pull over every half hour so he can "do his business". The ever so present stench of pee from that time you told him "that he would just have to hold it because you are not pulling over again" is a constant reminder that it isn't something he does intentionally to annoy you; he just has a freakishly small bladder, and no control over it. It is also, however, a constant reminder that he actually pissed his pants in the car and with the threat of exposing this story to everyone else you can get him to buy literally everything for you on the road trip.
If this guy is going with you, you might as well prepare both mentally and physically (especially if you have a sleeper amongst you) to ride in the back seat of the car. He knows every single rule in the shotgun rulebook and will always find a rule that makes him the shotgun no matter what. Luckily this also means that he is very serious about his responsibilities as the shotgun. This means making sure that you will always listen to the best road trip music available on radio, even if it means he will have to bring his own antenna. So instead of fighting it, maybe you should just let him?
There is never a silent moment in the car when this guy tags along. He can feel the mood in the car and instantly think of the perfect game to match that mood, whether it be I Spy, 20 Questions For The Professor or a third game that I can't think of but he totally could. It's not that you don't know these games, it's just that you choose not to keep them in your memory, because let's face it: car games are just not that fun. But that is also why this guy is priceless on a road trip; and yet VERY expendable in case you have too much stuff for the return journey.