I will have to have two stories.

Mummy and Daddy love each other and made the baby.

Mummy and Daddy had some help from Dr F to make the baby.

Each one, I guess, will be elaborated on as he gets old enough to understand a little more.

My parents told me that "Mummy and Daddy made the baby" when I was 3. I don't remember having a real understanding of "how" that occurred until I was older.

I don't think kids need to know lots of details - I think it's sometimes good to answer the specific question, but not offer extra information unless the child asks for it. Not withholding information, just not wanting to overwhelm them, iykwim?