He is probably not going to be comfortable sitting in a car seat even if it is completely reclined. I know when I was coming home the car seat was terrible and laying in the back of the car was really uncomfortable(miserable would be a better way to put it) because it was not designed for having somebody lay across the seats.

You may have a car that you can have your husband comfortably lay down while you drive. Whichever way you do it have pillows and blankets so you can set up a comfortable area to lay down. For that long drive laying down is almost essential.