Ok, think bunkbeds. What about attaching your son either to you or to some fixed railing in the car so he can't fall out of the bed. Perhaps something like this would work. We tried one with our 7 mo. old in a plane and it was essentially useless since she wouldn't sit still long enough to stay attached to one of us. However, for sleeping it (or something like it with a slightly longer teather) might be just the ticket.

Good luck,

Michael