apols v much skim read but fwiw I think a no-euphemism rule is good.
I did something along the lines of 'daddy put his seed, which is called sperm, into mummy's vagina with his penis. it swims upwards till it meets mummy's egg and then..
KABOOM! the sperm and the egg join together and make a baby!
Then pause, sweating. Wait and see if the sudden KABOOM! worked to er.. move things forward.
And if he does want further info, refer him to his father continue along the lines of " the baby grows bigger and bigger inside mummy's womb near my tummy. then when it's big enough, it starts to come out, back down through my vagina which is increeeeedibly stretchy. AND IT WAS YOU!