Checking in binders. Glad to see you've started a new thread 
I've been musing about your situation and have a few thoughts...
When you gave STBXP the deadline, did you give him a specific time? Assuming he hasn't made his intentions clear yet... what are your plans for if he decides to conveniently be out for the day?
I'd be tempted to send him a really clear message (by text, email and in person if necessary) along the lines of "If you have not made it clear by x-time, and are not here to tell me in person, I will assume that you have found somewhere else to stay and will pack your things for you. Do let me know where to drop them off for you as I'd hate for your precious things to get damaged out in the rain."
He might be hoping to avoid confrontation by simply not being around, thinking this could be the way for him to stay in the house or buy more time to win you around. Don't give him that opportunity.
I agree with others that you should try to stay put with DS and get him to leave if it's possible (for me, it wasn't an option in the end as safeguarding issues had developed). It would provide you both with the stability you need. As regards changing the locks... I'm sure I read somewhere that that might reflect badly on you as you're both joint owners. But it wouldn't hurt to have bolts on the doors and use them when you are home, especially at night, which would deter him from trying to sneak back in.
Should you need go to stay with your sister for a while, is there a chance of finding an affordable, appropriate rental for you and DS? Not to relinquish your legal claim on the house, but to ensure stability and a new home for you and DS. I'm sure others will be able to advise if this would be a good idea or not.
Thinking of you tonight binders. Hope you and DS are holding up okay 