i would just try to think about what his routine is - when we travel we stop every coupl eof hours for the girls to have a play and snack/meal, so with a smaller lo i would do the same - when it is his A time i would stop the car somewhere that looks nice and go for a stroll, have him engaged and looking around, play with him on a picnic blanket in a park etc. do his feed and so on in the light, then do your wind down, pop him in the car and drive for the next 2 hours hoping he naps well in that time. when he wakes from his nap plan to stop for your next break at his E time and do the same (and make sure you guys eat too at those stops so you are not having to take him out of the car when you suddenly need to eat and then disrupt a nap when you don't know how each section wil go) HTH.
