Stepson influenced by his mother

My 17-year-old stepson has been really sweet and was very happy when me and his dad starting dating two years ago. His mom however got into a crazy blaming place when she figured out I was the new girlfriend of her ex because we use to be in the same group of friends many years ago. Me and my boyfriend did not date or have any romantic relationship before we both separately ended up as single Three years ago. To the point: The ex is influencing the teen son to the point he does not want me to be living with his father and makes a lot of drama around this. Our other 4 children are positive to us. So can you help me with an intentional model for this circumstance?
C: Me and my boyfriend are living together
T: Son is making a lot of drama but he is influenced by his mother. He is allowed to act however he wants, I will stay calm and loving
F: Relaxed, this will work out
A: Loving
R: I’m making a peaceful home for my Family.

It’s so easy to become stressed and hurt by the situation but I’m really working on my intentional model and would love to get feedback.