I had that same issue, on the way there (4hr drive) I had him sleep first nap @ home, but we had to leave in the morning on the way back so we waited till right before his nap time, i sat in the back and when he got fussy enough to fall asleep i put a blanket over the car seat and white noise from my phone (downloaded a baby soother ap) tucked that into the car seat and he slep an hour and a half. He NEVER sleeps in the car so it worked, maybe ether could work for u.
