I hope to spell things out clearly to him that I have no interest in being involved with my mom anymore, and hope he respects that.
Another possibility is to simply tell him you want a relationship, and you will not discuss your mom.
I was estranged from my father for 7 years while being in contact with my mom. I had to go through different iterations to find a way that worked for me. At first, she set the terms and that led to nothing but kicking my emotions into high gear. The overall message was to get back in line.
Initially, there was a pattern to her visits. Day one was light-hearted, happy, fun. Day two she would begin hinting. Day three she would be in tears about the estrangement. By the time she left I would be an emotional wreck.
For me, I learned to not take any bait. I didn't discuss what I was doing because those conversations often opened the whole can, leading to the same outcome: me an emotional wreck. I simply learned how to recognize bait and let it dangle.
My mother's codependence took me a long time to fully understand. She is much more covert in her manipulations and at the end of the day, her goals are different than mine. I welcomed her into my home but I kept doors closed, and while she continued to hope for a reunion, those conversations tapered off and they definitely did not go through the full cycle once I saw my role more clearly.
I was also careful what I shared. I'm certain that the reason my mom was *allowed* to have a relationship with me is because information she gathered meant my dad could behave as though he knew what was going on in my life. He is very narcissistic with a large network of shallow acquaintances. Being estranged wasn't a good look for him.
One time my mother was visiting and she used an old computer. She's not computer savvy, and she left open an email to my dad that she wrote while visiting. It was filled with lies about things I said. I'm guessing she was trying to put words in my mouth that would appease my dad and make him feel better, maybe so she was allowed to be in my life? I don't know. It was a turning point, realizing that she is a kinder accomplice, but she is still an accomplice.
It feels better knowing that I cannot fully trust her, to be honest. It helps me set boundaries and not be suckered into thinking her loyalty is with me. That helps me manage our relationship and have a more realistic view of where her allegiances lie.