Answer
In a sense, his huge acceleration caused the force that pushed the blood to the back of his head.
His blood tended to stay in place by Newton's first law, and his body and head were accelerated forward by a force from the seat and headrest. When the inside of his accelerating head and body met the stationary cloud of blood, it pushed it forward (action).
The reaction force was the blood pushing against the inside of his head/body.
