Relationship breakdown with our adult son

My son is 25yrs old, he has always been different in that he is very clever but struggles in social situations, he fixates on certain things usually hobbies, collecting things etc, swapping from one to another. He has a fiancé and little daughter, they all live together and he works full time.

I also have a daughter, their father and I separated when the kids were 8 & 5yrs old as the relationship was volatile, looking back now I believe his father is on the spectrum too. I met my 2nd husband around a year after my break up, we have been together ever since, providing the children with a secure and loving home. 

My son was diagnosed with BPD a few years ago and then started talking about the trauma of his child hood being the cause, this has caused a lot of upset but I have tried to support him but that has become very difficult over the last year and the relationship has broken down, he doesn`t speak to any of us and we can no longer see our Grand daughter. I have tried a number of times to communicate with him but he has ignored me.

I went to his home as I was desperate to sort it out, my son wouldn`t let me in the house and tore strips off me on the door step, accusing me of not being there for him and saying he has been misdiagnosed with BPD and he is actually Autistic and said I should have known when he was growing up. 

He used to be angry at his dad because he wasn`t great when they were growing up, he let him back into his life but constantly reminded him or had digs at him about how he wasn`t a good dad. He has only in the last year started accusing me of being a bad mother, he accuses me of being narcissistic and gas lighting him, saying I am toxic, false and judge people, all of which are not true. 

His sister has tried to contact him, even reaching out because she was struggling with her mental health, but instead of being there for her, he pushed her away and told me it was my fault both my kids had issues. I just don`t know what to do, it`s ripped our family apart and I would give anything to fix it.