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He is asking because we had a friend who died and he understands that he cannot visit him anymore. We talked a bit about death and being born and he asked where he was before he was born. And I told him some but would like to be able to tell more clearly. And found out that other then, there is a part that stays and that you had other lives perhaps and that no you do not come to the life as you are now again, there is not a lot I can tell him. And that my understanding is lacking a bit to be able to tell him more (clearly).
What I would do, I think, is first to ask if he remembers anything from before he was born. Perhaps ask if he remembers being big or a grown up, when he's in a chatty mood. Many kids 3-5 do.
If he does remember, that makes your life a lot easier, and you can just tell him that your friend went somewhere else to have another life with another mommy and daddy, just like he did when he got born here. And perhaps that some day, he's going to leave this life and get born somewher else, too. You can explain that this is cool, because you get to do lots of different things this way.
If he doesn't remember, it's a little more tricky because less common ground. I'd probably explain it with a (computer) game metaphor, if he understands those a little bit. This life is like a game. A really, really big game, with lots of people and lots of adventures and all sorts of rules like you have in a game. You can say your friend stopped playing this game of being alive, and the rest of us keep playing without him, because he went home. Some day you and he will stop playing the game to and go home and maybe see your friend again.
If there's sadness, you could explain that it's normal to feel sad for a while when you lose someone or something (just when you lost Mr. Fluffy or Choochoo Train last week), but that you can also be happy that you met this person because there were nice times.