The poor little soul! What triggered this particular worry? did he read or hear something related?
I posted this comment in another thread but will paste it here for your consideration:
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Believe it or not, Lion King (yes the Disney animation) is an excellent learning tool for explaining death.
Young Simba: And we'll always be together, right?
Mufasa: Simba, let me tell you something my father told me. Look at the stars. The great kings of the past are up there, watching over us.
Young Simba: Really?
Mufasa: Yes. So whenever you feel alone, just remember that those kings will always be there to guide you. And so will I.
I will make another important point about life and death. Life is infinite (circle of life) and knows no end therefore, death is another form of existence, allowing one to change from a physical being to a spiritual one.
Mufasa: Simba . . .
Simba: Father?
Mufasa: Simba, you have forgotten me.
Simba: No. How could I?
Mufasa: You have forgotten who you are, and so have forgotten me. Look inside yourself, Simba. You are more than what you have become. You must take your place in the Circle of Life.
I hope this helps!