Part of the reason he becomes nauseous is because he expects to, once he is put into a car. You need to break this pattern by leting him experience car travel without the 'car sickness' / nausea. So bring him on very short trips, starting with 5 minute journeys and working up to longer & longer trips. Also letting him see out the window while travelling (sit him up on a cushion) will help ease the nausea. If you can source 'Bachs Rescue Remedy', a couple of drops in his drinking water the evening before & morning of the day he travels will take the edge off his nausea. Good Luck