Hello jraya and

Coping with suicidal statements sounds beyond stressful. It makes a lot of sense that you want to be both considerate
and effective when he says those kinds of things. This group is a good one for finding others who are choosing to stay in challenging relationships and who want to be considerate to and effective with their partners.
It is good to hear that he's in therapy. How long has that been going on, and do you know what kind of therapy it is (i.e., is it DBT, etc)?
Are you in any kind of therapy or counseling for yourself right now?
...
Should I talk to him when he is back to himself and tell him that if he threatens that again then we need a better plan? Like, option a: he has to text his therapist or I get to? Option b, I need to take that seriously and we take him into the hospital? I don't want to push him to do anything crazy! I know he gets feeling he would rather die then work through things with me but part of me feels like, if talking things through with me is that difficult then we shouldn't be doing this partnership? I need to feel like we can resolve stuff - this is little stuff in my mind (and he then says 'why do we even need to talk about it then?' 'why can't you just let it go?') It drives me bonkers. I want a boundary. I want to feel like I can communicate and not get shut down with that kind of threat. It's been normalized and I hate that. I know he is down but I also want to be respected. Any help is much appreciated.
BPD is a big challenge in intimate relationships, as it can be thought of as a "relational disorder". pwBPD have extreme difficulty managing their intense, varying, and out-of-control-feeling emotions, and those feelings and emotions can be stronger the more intimate the relationship is. Shame is a big factor, too, as well as issues with feeling like an abandoned child, but also wanting to preempt that feeling by "abandoning" the self or partner first. Couple that with how pwBPD seem to have few tools, or low skill development, with managing conflict, and it's a lot to cope with.
What's clear is that it takes some pretty non-intuitive tools and skills and approaches on our end, plus working on ourselves, for our relationships with pwBPD to have a shot.
In a "generally normal" relationship (and there's a broad range there), what we're told is that "talking about the conflict that happened" is how to work through it. "If we could just talk calmly about what happened, then we could get closure and move forward". Bringing up the past is often not effective, though, when trying to stay in a relationship with a pwBPD -- the shame factor is pretty huge and debilitating. What might be more effective is focusing on what you can control on your end, versus trying to have conversations with him that he probably isn't capable of really having.
The good thing in this situation is that there is a lot that's 100% under your control.
One thing that's under your control is that you are allowed to call the suicide hotline even if you aren't suicidal and even if he doesn't want you to. You can describe your situation to them and ask what they would recommend in your case.
Another thing that's under your control is your personal
boundaries and values. Boundaries are rules for ourselves (not for a relationship, not for other people) that are totally under our control and that don't depend on others cooperating, agreeing, or thinking our boundaries are nice, good, or needed. We can often get confused into thinking that things are boundaries that really aren't, and then get demoralized when "he didn't respect my boundary" or "the boundary didn't work".
I'd encourage you to read through that Boundaries workshop and let us know what stood out to you, what seemed do-able, how you might phrase your personal boundaries?
For example, just like so many of us here, we want to feel like we can communicate and not get shut down, and we want to receive respect from our partners.
Where the rubber hits the road is really facing into if those are under our control when phrased that way.
"You should treat me with respect" is a demand.
"I choose to stay in relationships with people who treat me with respect" is a boundary.
Similar with suicidal statements:
"You should call your support" is a desire.
"When I hear suicidal sounding words, I will call your therapist for advice" is a boundary.
Ultimately, I think you're on to something that boundaries are going to be one important tool (among many others) in your new toolbelt for more effective coping with challenges in your relationship. I'm looking forward to learning more of your thoughts on the topic...
kells76