I begged her not to call on Saturdays so early, if not for me, then for my roommate. Her response? "I NEED to talk to you, and it's the only time I know I can reach you." She seemed oblivious to what my needs were--to sleep in on Saturdays, to be respectful to my roommate, to keep the conversations shorter, not to feel like she was guilt-tripping me, etc.
In her elder years, BPD mother's phone calls to me were mainly need based. Usually an emotional need. She'd even say she needed to speak to me so she could feel better- if she felt anxious or upset.
This is different from a relationship with an adult child. An elderly parent is a unique situation, and while we needed boundaries with the BPD behavior, being available for emotional support when we could be was a reasonable expectation to us. If she called, we'd answer if we could be available.
We also called her on a regular basis, but often when we did- she'd not want to speak. She'd say she can't talk, she's too busy, and hang up. If she was in the mood to talk, the conversations were mostly light- about a movie, or a book, or something not emotional. When she called "needing to talk" what she needed was for us to listen and to validate.
One thing to ask yourself when you call, who has the need? If you are calling because you feel you need to speak to your son, maybe he doesn't have a need at that moment.
Adult children have the developmental task of differentiating from their parents, and I think in particular, for a young man, their mothers. Your son may be experiencing dual needs. One being- he may still feel dependent on you, but he knows this is not the norm for an adult man, to be "needing his mother" and in fact, may be seen as humiliating, and so he may also swing between the two- one needing you, one pushing you away due to preserving his self image.
You still want this relationship with your son, but it may be that it can be on his terms only. We wish for some form of reciprocity with a family member- but with a pwBPD who is overwhelmed with their own feelings- this may not be possible.
So, with BPD mother, if we called and she didn't want to talk, then that was how it was. If she did- and we were available then we talked but we let her take the lead on what she wanted to talk about. When your son calls you, he wants to talk- that's still contact.