only read OP and first few posts.
I think you need to tread carefully here.
I fully understand your position re: your first child etc. but there is already a child in the family - your dss.
you are, I assume, looking to create a new blended family - you cannot hterefore exclude your dss.
how does your dss feel about the new baby?
I do have sympathy gor your situation, but it is one of those htings that happens when you are in a step family.
I had my step childrne with me (as in staying with us for contact, not at the birth!) for both dds. not necessarily ideal, as dh obviously had ot share his time. but as other posters have said - what woud you do if your dss was your ds?
there is a lot of talk about wanitng things ot be "right" about the birth of a new child, and people can get quite precios about it.
basically, it hurts a lot, and then your time in hospital is pretty boring - the more visitors the merrier is what I say (and I didn't have easy labours/nice time in hospital either time)
maybe your dss would love ot see the new baby, as one of the first visitors?
one of the best photos I have of my dss is of him cuddling a newborn dd1, maybe 4 hours or so after she was born (after a hideous 38 hours labour, which my step children were staying with us throughout). the joy and wonder on dss' face, to be holding this tiny little baby is amazing (dss was 13 at the time)
I think a lot of this "but I want my dh to be with me in the hospital and spend time with us" is overblown - you have a whole lifetime ahead to spend time together - not being just the 3 of you (which it will never be, your dh has a son already) is not going to wreck anything.