Hi AmericanInItaly and Welcome to bpdfamily.
It is incredibly difficult to know the best course when you are in a situation of explosive anger, especially when there are young children involved. It's hard to know what to do in the moment, as you witness abusive behavior happening to your child, which leads to that feeling of helplessness. Based on what you said, you want to stay married but are worried about your wife's BPD damaging your son. I completely understand your feelings, and you are definitely not alone.
The first things that you can do are to learn as much as you can about how not to make the volatility in the household worse. I recommend reading and posting on the Improving Board,
https://bpdfamily.com/message_board/index.php?board=6.0, as there are a lot of valuable communication tools on that page.
I remember watching my uBPDxw yelling at my S11 (think he was 8 or 9 at the time) as he lay on the sofa with a pillow covering his head asking her to stop. I was horrified, and I felt powerless to change it. Stuff like that eats away at you. Are you certain there you have no access to a therapist, even if it is not covered by insurance? My T helped me to see that I was not wrong to come to the defense of my son in those types of conflicts, even as my wife berated me for always taking his side. The T reminded me that I was the
only person functioning as an adult in those moments, and it was my responsibility to care for the children first. I developed a routine of interceding, drawing boundaries around speech (for both my son and wife), and I would remove him and me from the room or house when my wife was starting a dysregulation. I learned from the Improving Board how to create clear, consistent boundaries, did my best to validate my wife's experience and committed to returning in half an hour after working with my son. First and foremost, I made the commitment to be there for him. Sounds like you are being placed in a similar situation.
I am so sorry that you are going through this. I, too, worried about what would happen to both of my sons if I divorced. In the end, it is a very personal decision to stay or to go. Either way, there are people on this board that can help walk this journey with you and support you as much as possible. Try to go easy on yourself, and believe that you have the strength needed to protect your son during your wife's dysregulations. Learn what you can to not make things worse. It does and can get better. It will be up to you to decide if "better" is good enough. Hang in there. Lean on family or any willing ears that you can.