Answer
He is accounting for the centripetal force acting on him.
Work Step by Step
Since the car is moving in a circular path, it is experiencing centripetal force. This force is directed towards the circle. However, if the car is accelerating into the circle, the person is accelerating away from the circle in respect to the car. If the person leans into the circle, the inward leaning force and the centripetal force would prevent the person from losing control and flying away from the circle, due to lack of friction preventing motion.
