I'm an adoptive kid of a single mother with BPD (which I didn't find out until I was in my 40s), so I may be kind of a reverse member in a way

It's great that you are reaching out to others.

I was 2.4 when my mother adopted me, an interracial adoption. How old was your son even you adopted him?
When I was a kid, we hung out with other adoptive families. What I observed is that even in loving families, some kids tended to act out often likely to issues revolving around identity. A core trait of BPD is an unstable sense of self. Attachment issues often have their genesis in early childhood.
Another factor may be the family of origin (FOO). My mother was Native American, born on the reservation, and tragically died some years after I was adopted due to drugs and alcohol, so it's possible genetic issues might be in play.
How is your emotional relationship with him?
Turkish