It's just his point of view because of where he is at in life. Do you remember when you were a teenager and the first couple in your group learned to drive and you saw them in a car, driving? All "new" social experiences cause the same focus and is like looking for a new car and suddenly seeing just that make/model/color car?

I would just agree/diffuse your friend, pat him on the shoulder and say something like, "Yes, we can't wait to have a child," and/or tease him about "You'll be in just the right spot to advise and babysit for us!" Just agree with him in some way, "Yes, yes, we know you're the proud papa-to-be!"