Next time he threatens you with Court just send a text back saying something like yes, "I agree, I think it will be in everyone's best interests if we get the arrangements formalised through the Court".
And if he's not on the BC and there's no Child Arrangements Order in place you don't have to hand him over for overnights, especially if you're not happy with where he will be looked after.
Lots of MNers have said to apply for CMS, if you haven't already, I would really recommend doing this. Are you worried about applying for any reason?
I'd also recommend blocking him, his DP and his family from all social media.
Send him a text saying that you're happy to let him see DS but in the future you'd like all arrangements and discussions through an email address.
Set up a new email address, give it to him only and only check it once a week and block him and her from everything else.
That way you will get some headspace from them but you're giving him a way to communicate with you about your DS.
Unfortunately though, you can't make him want to see his DS.