My autistic child has gone through several phases of being fixated on death and dying, and said some extremely shocking things starting as a preschooler. It was absolutely surreal having these talks with him but it was important to him and I’ve always given him honest and factual, albeit not necessarily detailed, answers.
Some of the phases were curiosity about the biology of it and trying to understand how the society he lives in handles death and why - funerals, why we have flowers, what happens to the person after they die etc. He was satisfied with factual and age appropriate answers.
Some of those phases were on the face of it curiosity but really because he was extremely anxious about his own death and that of close family members and he was seeking a lot of reassurance.
And at least one of those phases was actually not really about death but he discovered that announcing he was going to die and the like had a certain shock value, especially with unfamiliar adults, and he was using it as a way out of dealing with upset or anger or an adult demand on him.
I’m not sure what your son’s level of understanding is, but I think while you might need to distract him “in the moment” if it’s a completely inappropriate time for that conversation, if at all possible you do need to engage with him on it and try and find out what’s behind it.
For what it’s worth my answer if my DS asked the question your son did would be “It’s very unusual these days for someone to die age nine, but yes it sadly does sometimes happen either because of a really bad accident or because of some rare illnesses. It’s not something you need to be worried about though, it’s very unlikely. What made you think of that question?”