Hi Explorer36,
Welcome and hello

How old is your son? Does he accept the diagnosis?
I went through a similar process with my partner's D19. She was diagnosed first with psychotic depression/anxiety, and then with bipolar. My therapist, in hearing me describe D19's behaviors, is certain she is BPD. It came as a shock to me and like you, I wasn't convinced. There are 9 criteria, though, and having 5 of the 9 (to get the diagnosis) means over 100 different combinations.
The most difficult of the criteria, from what I see with my SO's D19, is the identity disturbance and unstable sense of self. She has a sense of being dead and empty inside, as though there is nothing there, no being making her feel whole.
What are the descriptions that describe your son?