Me again,
I read your story and it is so similar to what played out with my husband and his son.
With the exception that the son is not BPD that I know of, although he certainly has NPD traits.
Dad was a great dad, he spent time with the kids when they were young. He participated in making easter eggs, gingerbread houses all that stuff. More than most guys I know.
Right now the son is 33, and had a baby girl in July. Dad and son are finally reconciling and speak on the phone regularly. We don't live near each other and I feel like that bit of separation is helpful. So - that is the good news.
The history is several years of estrangement. The tragic part is that the son was 16 when his mom passed from cancer. Dad spent 4 years looking after mom before she died with little to no support from her parents. Always treated like he wasn't good enough. Son was a compulsive liar and a thief and dad's efforts to deal with his son were met with resistance from not only grandparents but his circle of friends as well. Everyone believed son, he was charming and everyone felt for him because he lost his mother. Nobody gave dad the support he needed losing not only his wife but now losing his son also.
The stunts that son pulled were unbelievable. The way he was able to manipulate everyone around him was unbelievable. The fact that relatives and friends believed the lies and manipulation was unbelievable. Dad finally gave up, he didn't have the mental energy to fight this anymore, he was still grieving his wife.
In the end, the years of estrangement may have been a saving grace. Dad and son were each able to get on with their lives. No more conflict.
Things aren't perfect but they are better. The estrangement ended when son got married the first time. We were invited to the wedding and we went. Grandparents accepted me as stepmom for the first time whereas before they refused to be in the same room. No basis for this but whatever. That marriage didn't last and when son got remarried he chose a place and time that made it impossible for dad to attend. Almost like he was testing dad. Son displayed tears at the wedding about dad not being there. And once again everyone felt sorry for son, dad was the bad guy - again. Dad was hurt and frustrated but has let it go.
In the end I believe that what worked for dad was radical acceptance. He realized early on that he could not change things no matter how hard he tried. So, he made new friends, found new support (me) and went on with his life. He focused on other things.
I think that the other thing that worked - sorry to say - was the time apart.
I know you are hurting right now, and it is really hard. All I can say is that there is hope for the future. Right now you must feel as though you are banging your head against the wall trying hard and getting nowhere. I just want to say that radical acceptance and doing things for yourself work. Take a break from it if you can. Find something else to focus on if you can. Sometimes things actually get better when we stop trying so hard.
I'm just saying what has helped my husband go on and enjoy his life. And now, with bygones being bygones he gets to enjoy facetiming with his baby granddaughter.
I hope this was helpful.

R