Son calling step mom, mom

I'm feeling really upset by this. My son is 17 and I did tell him that I don't like him calling his step mom mom but he does it anyway. My son's dad and I don't get along. I was an idiot when I met him. We met at work 20 years ago. He said he was gonna leave his wife. He wasn't happy. But then wanted to work things out with her. We fooled around for a while. Then she got pregnant and he cut off contact with him. I found out I was pregnant and had to go through his family to Contact him. I ended up being forced to be a single mom and his wife clearly hated me. Our sons are 2 months apart. He did step up and take care of our son. I'm just hurt that he forced me to be a single mother. I found out last month that my son also calls his dad's wife mom and it hurts me. She's been in the picture since he was born so I get it but I do not like it. I've told him you only have one mom. Me. But I found out my son still calls her mom. I absolutely hate it and my ex won't correct it. I'm not sure what else to do.