When Adrian was 8 weeks, we did three and a half hour trip in the late afternoon that he ended up sleeping through. I really feared what his night would be like. But to my surprise, he went down normally and slept as if he didn't have that gigantic nap. (He is a 45 minute napper.)
Since then, he always sleeps well during the travel, being it on a plane or in a car, which I consider a blessing.
So, don't stress too much about it. He had 11 weeks to form good habits, one day won't spoil it. If he ends up sleeping for the most part, enjoy the ride. Keep him up a bit later when you get there and put him down normally. The next day, stick to his routine and he will be fine.