skypilot my dh always says if we were ever to split up he's want to live next door or find a flat just around the corner so he could be with the kids every day. Is this an option for you and your dh? I know a few people who do this. My BIL moved just around the corner when he and SIL split up and he was always fully involved with their son.
Otherwise, it sounds like you like him even if the spark has gone. If there's enough affection there perhaps it's not so bad staying together like flatmates. Perhaps you could even take other partners if you both agreed to this and it was out in the open.
I'm sorry you're in this situation though. You must feel quite trapped.