My daughter wanted to talk about death when she was three or four. I just remember telling her people go to sleep and don't wake up. I agree with you we don't have any proof what happens either way. She seemed ok with that one chat and didn't raise the subject again so maybe your son won't. I don't know what she believes now she's twenty but that's her business.
The next thing when he's about five will be questions about God. Fun isn't it 😄