OP this must be very hard for you.
Him going NC is simply a way of continuing his abuse now you have put a boundary in place over money.
You need to see this for what it is and maintain this boundary also.
This is a teachable moment for both of you. He needs to learn that his behaviour is not acceptable (in fact it's despicable).
You need to accept (as hard as it is) that he is simply not a very nice person and failing to maintain your boundaries and give in to him enables his sense of self entitlement and narcissistic tendencies.
Have you told your other children what's been happening?
I think you should. Firstly so they can support you in standing up to him, but also to be on guard for themselves.
Now he's not able to drain you of cash, he may well start on his siblings.
You have 3 children and sadly one is a bad egg. Sometimes it happens.
I have a friend who is lovely as is her older brother. Her younger brother is vile. Parents really good people who would do anything for all of them growing up. They are now all NC with him after years of abusive behaviour, to his parents and his siblings. My friend said, controlling and abusing other people seems to be the only thing he enjoys in life. That he can only fuel his self worth by proving he's better than the rest of his family because he can manipulate them - and so in the end the only option was to cut the fuel line.