Ex is upset because our shared son calls my wife mom now

I used to get my kids every weekend. My son is 14. Daughter is 16. My son moved in with me 10 months ago because he does not get along with his mom's boyfriend. We had to change our custody agreement because she has full custody of our daughter. I have full custody of our son. Funny thing on how our state handles child support in situations like ours. What they do is take how much we both owe. So for me is was 400 a month. For her it was 200 something. They subtract it and I PAY THE DIFFERENCE. So I pay $200 a month and she pays nothing. Idk why they do it that way because it's basically I'm financially responsible for both kids. She's financially responsible for one. They claim this keeps it fair and money from bouncing back and forth but it's just me financially responsible for both. She's responsible for one.

Anyway my ex and my son haven't spoken to each other in 6 months. They don't talk. She didn't wish him happy birthday. They refuse to talk to each other. A lot of this is over the boyfriend. I wanna say 2 months ago he started calling my wife mom. She started including him in helping with our baby and took him to do things for his birthday. My daughter told my ex that he calls his step mom, mom and she calls me yelling and cussing at me because why is her son calling another woman mom and that's not his mom. I said I wasn't making him call her that. He was choosing that on his own. She said she's his mother and his only mother. I said how? You don't financially help with him. He's living here because you pick your boyfriend, and you haven't spoken in 6 months. You haven't even called on his birthday. She said she's taking me to court for full custody back and hung up on me. If he wants to call her mom I'm not stopping him. And I love that they've built a relationship especially since the one with his own mom is strained. Am I being an ass?