I think that Jacob might tell his parents about what he saw the man mouth which is the phrase 'You're dead' and they might not believe him, ending up bring him to his destination instead.
I think Jacob might attempt to stop the car and tell the parents about what the hitchhiker had said and try to convince them to leave him behind, instead of having him for the rest of the journey.
I think that Jacob would try to convince his father to stop the car and get the hitchhiker to exit the car and drive away from him because of his weird behaviour.
I think that Jacob might try to find a way to communicate to his parents about what the hitchhiker mouthed to him, resulting in the hitchhiker leaving the car in the end.
I think that Jacob might attempt to tell his parents what he thought about the hitchhiker and what he hitchhiker mouthed to him, resulting to the hitchhiker leaving the car and walking in the heavy rain.