Any advice would be appreciated

Help! My adopted son is a 19 year old narcissist and he has put me through so much drama/trauma to where I am now in counseling to get some form of help. My son refuses to get any kind of help as he states that there’s nothing wrong with him. He currently started dating a girl who just turned 18 and is still in high school. They have been together for 4 months and in that short time frame he has control of her finances, who she associates with and keeps her on the phone 24/7 so he knows exactly what she’s doing, even while she’s at work. He told me they have found an apartment and are moving in together. He stated that once he moves out he will cut all ties with me and the rest of the family. He told me my job as a mom is done. I love my son with all my heart and despite what he has put me through I can’t let him go. I am so heartbroken that he is cutting me out his life for good. He says all he needs is his girlfriend and he’s all she needs. He tells me it will just be the two of them forever as she is also excluding her family from her life as well. Now whether that is true or not I do not know. He will not let me meet his girlfriend giving me excuses that she’s too busy. Last night he told me she doesn’t want to meet me because I do not like her which is false because I do not know her but just her name.
What can I do to save my relationship with my son? It has only been the two of us these last few years and we’ve built a close bond until he started dating. That is when his behavior towards me turned negative and we have become distant with each other. Has anyone else gone through a similar situation? Please help
Thank you