A.H.
I cannot sleep in a moving car, either. It's the motion--not motion sickness, just the movement of the car itself--that won't allow me to sleep. Riding in a car for more than an hour wears me out, but I can't sleep while we're moving! It may be the same for your son.
I would just plan to travel when he is not tired, and keep him entertained, plus make stops when you can to allow him to stretch his legs (and change him if needed).
Not much point fighting nature on this one, IMO. ;-)
--A.