His response to his triggers are escalating
Any thoughts on what might be driving the escalation?
I let him know I cannot continue in the relationships unless he gets some support, and he has ignored me since that conversation.
That's more of an ultimatum (unless you do x, I will do y) which can feel like control. His response appears to be to try and control you back (silent treatment).
How do I hold onto my boundary if he won’t speak to me?
Boundaries work best when they are things we have control over. Since he likely has no boundaries, you will have to put ones in place that work regardless of what he does.
Since he's in silent treatment mode right now, you have a bit of a break to focus on you. Are there things you enjoy doing that fill your cup? Silent treatments can take a lot out of us. It's important that you fill yourself back up when you are being drained by attempts to erase you.