I’m so sorry that you find yourself in this tremendously difficult position.
I can make a guess as to why your relationship changed so dramatically after the birth of your child.
Mortality—a child really gives parents a sense of the fragility of life, aging and its associated decline in strength, vitality, health, attractiveness.
That people who have BPD have an unstable sense of self might explain his shifting moods. And it seems likely that he fears the loss of his youth.
Whether or not your relationship ends, this article might have some useful ideas: https://bpdfamily.com/content/surviving-break-when-your-partner-has-borderline-personality