If this is all that you are seeing then I would like to congratulate you! Your son is developing right on track. He doesn't understand why his friends have babies and he doesn't and this is also very normal.
What you are seeing is your son moving into the pre-school developmental phases. This is when they begin moving away from mom and dad so to speak and begin to interact more with "friends" and siblings. If the behavior goes beyond the age of 5.5 to 6 years then you will want to have an assessment done to determine what attachment issues he may be experiencing.
The next time that he asks you where your family's baby is at just tell him the truth at a level that he will understand:
Well sweetie, babies are very special and when Jesus-God (whatever word you prefer that suits your faith background) gets a real special baby he has to find a special mommy and daddy and brother to take care of it and love it. When he finds the special people to love the baby he puts the special baby in the mommy's tummy. When it is time for the baby to be born the mommy will go to the hospital to have the baby.
There is no need to try to explain the sexual act of conception ...This is way beyond the scope of a child's brain development; But, it is good to let him know that you go to the hospital and the doctors help take it out.
I have found that it is very OK for the parents to tell the child that is very inquisitive and asks how the baby gets out to be honest without providing details that are to much for him to understand. The baby comes out through the vagina...End it
Whats a vagina... It's a private body part that Mommy's have so they can have babies...End it
If the question and answer session goes this far it will usually stop at this point... Children understand the basic concept of private parts and after their curiosity is satisfied so are they.
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