Is it possible that he has been exposed to some kind of violence (eg on tv) that has caused him to 'obsess' for want of a better word about death because he doesn't quite know what to make of it? Perhaps sit with him and ask him next time he says something like that why he suggests such things, but not in a way that will make him feel ashamed. Or perhaps ask next time he questions that sort of thing in the car whether or not he has ever seen that sort of thing before and what his feelings of it were (for example if he asks about others falling out of the car and dying and how you would feel, ask him how he thinks you would feel if that happened, or ask him how HE would feel if that happened).