Chillax
The kiddo has a way about him since forever. Whether he's playing, reading, drawing or watching tv, he would somehow slither down to the floor or sofa and continue whatever he's doing lying down.
He does that even while shopping with his mama and felt at home too when he was the only one in a horizontal position in a sea of other kiddies in the school assembly hall!
I think he would do it in the car if possible. Fortunately, he is still too young and required by law to be restrained and confined to his car seat.