I think he might try to secretly inform his mother about the blood on the hitchhiker's arm and what he had mouthed to him, and try to get his father to stop the car and make the hitchhiker leave.
I think he might keep talking or making noise to get his mother to constantly look at the back so that the hitchhiker will not have a chance to do anything that may cause anyone to loose a life.
I think he would try to alert his mother via the mirror, as his father will not believe him. The mother would then try to get his father to make the hitch hiker leave
I think he might either push him out the car or call the police and pretend he is talkin about the movie but explains the situation in the car.
He might try to inform his mother about the hitchhiker and try to get the hitchhiker to leave the car.
I think he will try to ignore what he said and be more alert of the hitchhiker. He might also try to inform the police about the hitchhiker.
I think Jacob will text his mother, so as to not let the hitchhiker know, to inform her of the situation and get her to make him leave the car immediately or inform the police.
I think Jacob would have tried to do something to the hitchhiker to disable him. For example, Jacob knows that the hitchhiker's hand is injured and uses it to his advantage by trying to jam his hand in the car door or window and knock him out for a while. Then he might call the police or tell what happened to his parents.
He may stay silent while trying to get attention from his parents. Making an indiscrete move would alarm the attacker, hence he has to rely on body language to alert his parents about potential danger.
I think that he might realise that "Rellik" is actually "Killer" spelt backwards and then ask his parents to make a stop with a reason like he needs to go to the washroom. He will then inform his parents that Mr Rellik is a person who escaped from the mental asylum.
He might try to alert his parents to try and get the hitchhiker off their car. He might have used some discrete move to do so, through texting, or maybe asking for a rest stop.
I think that Jacob might have alerted his parents to stop the car, but also to politely ask the hitchhiker to alight as he seemed very scary.
I think that he will somehow or rather try and alert his parents secretly about what the hitchhiker did and thus, surprise the hitchhiker.
I think he would try to warn his parents about the hitchhiker, who he considered to be a great danger.
I think he would try to signal his parents about his bad vibes from the man and try to find a time where he can speak to them alone and tell them about why he suspects the hitchhiker.
I think he would secretly tell his parents through a secret code.