As far as letting him take S on Friday - well, that is entirely based on what *you* are comfy with. I know for me, I would drag that out as long as possible.
I also think it is time to start making this situation as uncomfortable as possible for him. I think he needs a dose of reality on what a divorced dad's life is truly like. Meaning, I would put down definite boundaries on S that you are 100% comfortable with. I am pretty sure the courts wait until baby (at least in Ohio and Michigan) is at least 1 year old before they allow overnights.
2x4 time, said with much caring and hope for your situation.
Right now, your WH has it soooooo very very very easy. You are making this a walk in the park for him. Granted, the changes are for you, and for him to *see*, but he is doing far more than just seeing the changes but benefiting from them directly.