personally I wouldn't give a lollipop if he's in the back and DH is in the front, just because of some scary story I read.
Oh just another thought, if you can't do it overnight maybe you could set off half an hour before nap time, so that at least he won't be spending ALL his day in the car, he can have the morning to run around and have a nice time, have (hopefully) a nice long nap in the car, be grumpy in the afternoon, nice long stop for tea, 30 mins run around, and change into pyjamas, and then (hopefully) go to sleep for the last part of the journey.