I 51F do not know how to save relationship with my child 18M

I do not know if it's the right place to ask, but I am desperately in need of advice. I am the mother of a transgender son. He was a girl and is transitionning to a boy. He came out 3 years ago, but has only really changed with testosterone for a few months.

Our relationship is difficult to deal with lately. I worry about him with the bullying. The grand-parents on my husband's side have stopped acknowledging their grand-child. I try to ask questions to my transgender son to make the family come around the situation, but he lashes out at me calling me transphobic. I try to tell him I am not transphobic I am confused and need answers to explain in simpler words to the family. He tells me that I do not understand him, that my husband and the grand-parents are a lost cause. He told me that he doesn't want to hear my transphobia anymore and that he is cutting contact with me.

I do not understand what I am supposed to do. I try my very best! I call him my son when he comes to my job, I use his right pronouns and I am trying to learn his new name. It just seems that I can never get it right, it keeps getting worse when I apologize. It feels like I won't be able to save our relationship and I don't get what I can do to not have this happen.