@MrsT777 I really don’t think giving her blessing for the relationship is a good idea, She will trip up very soon she’s already asked him to kill his own mother, I’m not sure how worse it could get!
It’s strange but I have a feeling perhaps a little like the OP is feeling about her son - the son is feeling he can’t end it - the OP is feeling like she can’t be firmer about saying that he should not see her - and I’m feeling from a distance please OP tell him no way.
You don’t have to physically force him. You don’t have to threaten to kick him out. You just have to calmly but firmly say this is so serious, that this ends now. That’s it. No discussion, just calm, firm, strong.
If he starts to rail, just repeat, very calmly, very firmly ‘this ends now’. If he still rails, suggest that we all have a week with him respecting the no contact, and then talk again. Who pays for his phone? You, you can take it. Who pays for the internet? You. What if he leaves the house? You ask him very calmly not to go. If he does, you call the police to say he’s left, they probably won’t do anything, he’ll eventually come back.
But when he does, he will have this strong voice in his head that is saying ‘end it now, it’s not healthy’ and eventually that will make a difference. It is much, much better than his any meeting up with parents and you, or round and round discussions. That is all feeding into the drama and abusive which feeds on attention.