I recently posted on r/Relationships about an issue I had with my ex-wife’s BF and my kids

I’m posting this here because I feel that I have a more pressing matter with one of my sons than with his sister and brother.

Within the last few months, my son has become more and more distant from me. He's 13 and I realize that he's going through a lot of changes, but it's heart breaking to be treated the way I am lately. He shows me an utter lack of respect. He constantly uses profanity, makes crude jokes, and calls me names when he gets upset. Just this past week I was called "dipshit" along with his sister. This isn't the way I raised him, and I worry that this kind of behavior towards me is influenced, if not outright condoned, by his mother's boyfriend who has a searing hatred of me. My daughter tells me about the things he says about me to her, to her mom and her brothers. I can only guess that this continues on a more personal level with my son in question. This behavior has also coincided with him wanting to spend more time with his mom's boyfriend as well. This has extended to him not wanting to see me on the days I have scheduled to be with him, either to do something with the boyfriend, his kids, or to just stay home.

I realize that this kind of problem can only truly be resolved through family therapy and I'm trying to get his mother to agree to it. The problem is that she's dragging her feet. She doesn't see it, so she can only go off my word and his. She doesn't approve of his behavior towards me, but she's also not willing to help matters. For instance, when I explain that my day to be with them (I only have Sundays scheduled to be with them for 9 hours that day) is my day with them and that they should be with me, no choices, she says she's not going to make him do anything he doesn't want to. And if I talk to him and tell him, it's construed as me giving him a guilt trip. When I was younger, I don't remember being given a choice if I wanted to go see my dad. We just did.

This is so hard for me. My son used to be the most well-behaved kid. I still see glimmers of it from time to time, but for the most part I feel like we're not only drifting apart, but he's also doing a full on retreat from me.