hi, just got back from a week vacation with the kids, the baby is 10 months and we drove about the same time. well, first, i would really try to plan the time you leave. try to feed him and then go early in the morning, and hope he doesnt fall right asleep. maybe even sit in the back an hour to play, then move to the front and let him eventually sleep. when he sleeps, do not stop no matter what. its just too risky for him waking up.
then she woke up, and we wouldnt say anything at first. she would play, then want out. i would suggest when its time to eat, to stop somewhere, feed him, then maybe sit a little bit. if we put her right back in after eating, it was tough. but if we sat at the rest stop awhile, she went back in no problem. then hopefully, he will play a bit, and might even take another nap.
so we fed, played the carseat, slept, fed, walked around, play in the carseat again. then got cranky awhile and would cry on and off. stopped again, fed again. then eventually went back to sleep. it was fine the majority of the trip, but later on, it was more difficult. just bring lots of toys and only give one when he def doesnt like whatever he has.
dont worry about naps, it will be too stressful and he isnt going to sleep exactly the same, esp being in the car. you will be fine as long as you expect their to be some difficulties and dont stress about schedules. its not in your control. good luck