First off, my sympathies and condolences. This is the sort of situation that I suspect all of us dread, so kudos that you're proactively seeking to explore how to proceed.
If you're looking for a compromise from your OH, then you're going to have to make an opening "offer" and let him know what you want and need. Perhaps I'm being overly optimistic, but I'm inclined to interpret shutting down as either not realising that he's not responding in the way that you need, and/or not knowing how to. I know from experience how incredibly frustrating this can be, especially when it feels like you're communicating loud and clear, but unless you take the initiative I don't see how things can move.
If I'm thinking about it from your OH's perspective, then I'd be feeling pretty scared and helpless at the moment, which may be another reason for the shutting down. Showing that you want a compromise that involves staying in the relationship (assuming this is the case) may help ease any sense of insecurity and open up a bit of space for discussion (and yes, this is incredibly speculative given I've never met either of you!).
Good luck!