Personally, I find this indicative of a great imagination. I may be totally wrong here, but here goes.
I think that your 7 year old son may have come to realize that life in some capacity is finite, that existence can be tenuous, and that what we have can be "taken from us" at any time. This is called "existential anxiety", and everybody goes through it. A lot of people deal with it by repressing it (not always the best way to deal with fear) but some can't (fear of flying, elevators, other phobias). This may also be a kind of "catastrophizing" (although most studies of pediatric catastrophizing have been in relation to how children experience pain).
Does he happen to be gifted or an emotionally intense child? This tends to happen more in this group.
You're not describing anything worrisome; more commonly it's puzzling or concerning to the parent.
If he is experiencing some existential anxiety, I would do what you're already doing (analyzing risk). I wouldn't only do it verbally, though. I would definitely encourage observation. For example, with his "What if the stove catches fire and the house burns down?", I would walk him over to a window, and (kindly; he might be genuinely worried and wants to be heard) ask him to identify all the burning houses he sees (hopefully he can see a lot of houses). When he answers "none", ask him why that is. If he wonders if the car will blow up, ask him to count all the burning cars he sees from now until he reaches grandma's house. Give him assignments, maybe even as a way of warding off exasperation.
Another approach (this is particularly important in catastrophic thinking) is to introduce intermediate steps. He jumps to the extreme anyway, so introduce real possibilities. A flat tire or running out of gas is a lot more common than an exploding car. Floating in cold water alone for an hour or two is a possibility if the pilot has to eject. Being stung by a jellyfish is a more likely possibility. Ask him what sharks usually eat. Let him know that sharks actually prefer fish; if a shark bites a human, they typically bite, then let go after realizing they’re not eating sea animals. Lots of things can go wrong and interrupt making dinner; you might experience a power outage, or the stove might break down. You might spill the food accidentally. What will happen if this occurs? Sandwiches or cereal for supper!
If you're concerned about this, maybe you can minimize his exposure to violent scenes on TV or in movies. Pick some books for him where unexpected things happen and the protagonist overcomes the obstacles.
If this persists, or he has trouble sleeping or other signs of anxiety or stress, it's a good idea to talk to his doctor.
What is Catastrophizing? - Cognitive Distortions
a humor based website on catastrophizing