I think the above advice is great. Act like it's no big deal--this is a tactic strictly to aggravate you and if you don't bite he'll abandon it in short order. It's a shame that it is inconvenient for the kids, but as noted, they can sleep in the car. I used to go on four-day family car trips when I was a kid, and I did a LOT of snoozing.
Hugs,