Rb, a few thoughts.
First, I wouldn't try to engage him any more with any questions about his plans. I think that is just stirring the pot at this point. It's likely that he has no plans because he doesn't think you will go through with your boundary so asking him just gets him agitated.
It is common for people to react strongly to a new boundary. Cloud and Townsend, in their book, Boundaries, recommend that we be prepared for this. It's something new, it's changing the status quo, and people don't like change. So they are likely to push back hard, and often, our difficult children ramp up the pressure and the threats. Often, they don't carry through with them, but you should be prepared in any event.
Once I found out difficult child was stealing from me (he was living here and had just gotten kicked out), I went to Home Depot and added slide locks to the doors and changed the code on the garage door boxes. He was furious. It nearly killed me to do it, but the day I found him at the back door trying to get in, I knew I had done the right thing. He hasn't been able to steal from me since.
If your difficult child mentions his plans again, I would tell him that you will immediately call the police and get a restraining order. I know he said he's ready to go to jail, but that is likely talk. And don't react to the "jail" thing. He likely knows that will upset you a lot, but I would try not to show it. If he wants to put himself in jail, he can certainly do that, and RB, that might be the wake-up call he needs.
I would not stir him up, just speak calmly and quietly whenever you do speak to him.
Ramping up is very common. And if our difficult children see that it's getting to you, they will keep on poking you in that same spot.
It's too bad it has come to this and I know your heart is hurting. Often, things have to get worse before they get better.
Keep sharing, and we can certainly be a sounding board for you. Take precautions for your own safety. Warm hugs. Keep moving forward.