They have ice chests that run on 12v may be a place to start. Unless he's gone months at a time shouldn't be an issue, the vehicle has to be comfortable for him so should be comfortable for the cigars.
I havent herd about anybody using them but aren't some of the 12 VDC travel fridges (like the one above) thermoelectric??? that would work if the hey had the right thermostat , toss in some Bovedas and done!!!!
But as @Boudie said if temps are not extreme ther might not be a need for anything but a good sealed cooler