I think you can solve this with logistical changes - new, nice mattresses for your family sleeping space, an inclusive setup (maybe mattresses on a shared tatami mat, or arranged around a central feature such as a table?) and new rituals. Maybe your partner reads to your son in his bed and snuggles with him and moves to your shared bed (in the same room) once your son is asleep? With a new baby, one bed for big kid, one bed for Daddy, and one bed for Mom and baby is really not a bad idea.
I really like this suggestion. DH & I have our mattress on the ground and DS's smaller mattress butted up against it. He usually sleeps in our big bed anyway but at least it's there as an option (and, I suppose, would be looked on more positively by anyone who's anti-cosleeping, i.e. CPS if you're worried about that). Friends of ours have two big mattresses right next to each other, one ~8" higher than the other, & often the kids sleep on one & mama moves onto their mattress to nurse, or dad sleeps with one kid and mama with the other 2, etc. -- it allows them a lot of flexibility for various situations. It's still a big family bed basically but something like that might be a workable compromise, plus give you more space for the new baby.