Well, tell him the truth, I suppose, Dad is right, sometimes they have to make a hole in the mom's stomach andTAKE the baby out. Most times the baby will come out by himself, between your legs. If he questions whether there's enough ROOM (and I doubt he will) for a baby to come out, tell him mom's body CHANGES to let the baby out, then goes back to normal!
There is no harm in using correct terminology if you're comfortable with it, the more matter of fact you are with your answers the less mystery he'll feel and the fewer questions he'll have in the future.
It is after all a very beautiful natural process, it's only icky and weird to kids if WE make it icky and weird.
Good Luck and congrats to you and your husband for your growing family!