Hello, bowen
I'm sorry to hear you are suffering. It sounds like you know he is suffering too. It is a blow of mixed emotions when we hear about BPD and it fits. We are sad they are sick, it is confirmed, but that explains a lot, so there is some relief, and hope.
We know when we know, friend. We can't go to the past and do things differently. We couldn't know.
It seems like you think you can tell him. We are never sure. But if he things something is wrong, and he doesn't know, it might give him some direction in his life. You can point him to a video on youtube, or a book... .And say that you saw/read that and you were reminded of him. Be very careful, because there is a huge negative stigma about this illness, and if he thinks he has "something ugly and incurable and toxic to others... ." You know it can break him. But if the disorder is explained as a high sensitivity troubled by trauma, and that he has no fault in this... .That he had to develop some defense mechanisms to survive, but those defense mechanisms are now making his life harder... .Well, it's a different thing.
For me and my Gf, the book "the buddha and the borderline" made a great turning point. I can't find a video I was thinking about.
The key is to define this as what they feel, not by what they cause to others. They are already very ashamed about how they treat others.
Missing someone you love is very very painful, I hope you get him back, if that's what you want. But I get the sense that you would like to help him even as a friend. If you work on understanding emotions and improve comunication skills, it won't go to waste anyway, it also give us a better understanding of ourselves. Please, take time to read the articles of this site, at your own pace, and keep posting so we can help each other.
