I can hear the distress in your writing MissT. You aren't alone, but now have a whole community of people who can understand.
My father had a meltdown yesterday regarding my boundaries. He becomes belligerent when I set them or hold him accountable to what he said he'd do.
Miss T, can you share with us what you said to him that "set him off"? Maybe we can help. One of the things it took me a while to figure out and accept once joining this community, is that the normal ways of communicating with the rest of the world don't apply to a pwBPD, because all the "normal" things we do, tend to make a BPD more reactive to us, which makes the BPD behaviors worse. So all the strategies you use with co-workers friends etc., you probably won't use with your dad. It takes a while to get the hang of new strategies such as SET, asking validating questions, and
not JADEing. But in my case, I've been learning and practicing this for close to 2 years, and it really does make a difference. It doesn't change their BPD, but it's a healthy way for us of reducing and managing the conflict that results when interacting with a high conflict personality.
Projections, shaming, threats to end the relationship and financial ties etc.
Yes yes yes.
These meltdowns are particularly triggering of ptsd for me and it only makes me want more distance.
yes yes yes
I cannot give him what he wants (which is constant involvement and emotional support from me)
You are right. You can't give him what he wants. I can't give my mom what she wants either. None of us can, because it's impossible. And trying just puts us in a black hole that we can never get out of. So the only thing you can change in this situation, is how you react to him. This is where the communication tools such as SET, asking validating questions, and not JADEing come in. Also, most of us who are children of BPD's have been trained to be caretakers of their emotions. They want us to feel their pain, and be responsible for it. People on this site were kind enough to point out to me, that it was time for me to separate my feelings from my mom's. Does this resonate with you?
I don't know how to move forward. I'm feeling overwhelmed and scared. These flareups trigger deep childhood wounds for me and it's really painful.
It can get better Miss T.
Have you read "Stop Walking on Eggshells" (SWOE)? A lot of us start with this book. I've read quite a few books which have been super helpful. There's a book list on this site which you might find helpful.
Last night he said he'd rather not talk to me at all then every two weeks (which is about how often I call him.)
Sigh. This is predictable...
There are things you can do to manage this. Personally, I would suggest starting with SET, asking validating questions, and not JADEing with him. And be patient with these new tools, as Rome wasn't built in a day, and it takes us a while to get skilled in using these tools. But they really work.

Meanwhile, be kind to yourself, and by this I mean engage in lots of meaningful self-care. Give yourself some separation from him for a while, to let yourself recover from this latest conflict. Feel yourself again, and stronger, before you venture interacting with him again.