I agree with CC43's scenario, that stepson is trying not to antagonize his spouse. So many of us here experienced the demands of a spouse to blacklist our families. And living day in and day out with such intense pressure and repeated interrogations can break a person, just like Stockholm syndrome.
I've posted before how confusing it is when someone we see as logical and sound mind, even who has a moral compass, says and does - irrational and hurtful things, because to me, my father was the stable person in the relationship. And yet, as if he were somehow spell bound, he'd do and say things that were out of character for him.
I do believe it is a form of Stockholm syndrome, and also somehow, an extreme form of enmeshment. My own theory is that- my mother's BPD thinking and perception, and logical reality, could not co-exist. Even for me, being around her, listening to her perspective, it was hard to know what was real and what was not. She would talk to us, for validation- and press us to affirm her thinking. Sometimes she'd say things that were real and others so close to reality, that they were believable.
Borderlines are not completly psychotic, and so it is hard to discern if what they say is grounded in reality or not. I know someone with schiztophrenia and when she tells me that someone is shooting radio waves at her, I know it's not real, but when BPD mother says "your aunt insulted me"- maybe she did, maybe she didn't. It doesn't sound as far fetched, but it could be just as untrue.
My BPD mother would also put people up to carry out her wishes and the main one was my father. So she'd get some idea in her head and pressure him to carry it out. When BPD mother wants something, she persistently pressures and escalates, and that need has to be met. Dad would eventually give in, just to get her to stop.
I think my father eventually just capitulated to keep the peace and eventually, the two of them seemed to act as one person. Two separate wills can not co-exist in these circumstances. BPD mother's feelings, and wishes predominated.
But he wasn't a hostage. BPD mother was the one who was completely dependent on him and yet somehow she was the one with complete power in the relationship. This took both of them. He had choices, (albeit difficult ones)- but he also had the role of enabling partner in this situation and he was still responsible for what he said and did, however, if he didn't go along with her, her reaction was extreme.
It was a choice of consequences. We kids, his family members- we weren't going to react like she did- which included destructive and harmful behavior and under extreme pressure, Dad would just do what she asked.
As to your stepson's messages. In the era before internet or cell phones, whatever I said to my father was shared with my mother. If I called him on the house phone, she was listening in on the extention. When there was internet, every email I sent him or he sent me was also seen by her. Sometimes I didn't know who was "speaking"- him or her as they both sounded the same.
I'd be willing to bet that your stepson's wife sees every message on his phone and is either telling him to send them. When he says these things to his sisters, or father- I think she is pressuring him to say them. Does he believe what he says? I don't know. It's possible he's so stressed and confused he doesn't know either.
For those of us who are not disordered, it's unthinkable for someone to cut off a parent, child, sibling- for no real reason. In some cases of serious abuse, addiction, criminal behavior- it may be warranted, but this isn't the case with your H, and it wasn't with anyone in our family either, but it seems to be a frequent situation in BPD relationships. Why, I don't fully understand.
I know your H is hurting. I think this is one of the more hurtful outcomes of these relationships. It's understandable that he has these feelings, but what I would suggest is that he doesn't react back out of them. Because it can cause more relationship harm than good. The step son is already in a highly emotionally difficult situation, it will only confirm his unfounded beliefs.
From my own experiences, I would suggest your H "quietly retreat" - for his own emotional well being. Don't make any grand statement of "you are cut off" to his son. Just be quiet, don't keep trying to communicate with him for now, because if it were with my parents, any communication to him is filtered through his wife, and may be deleted, used as evidence, whatever. If the son reaches out, ever, reply in a calm and non judgmental way but let the son come to you, if he decides to.
However, this is just my suggestion. I am not a therapist and so IMHO, your H needs counseling and professional advice on what to do. This is emotionally painful. He needs to take care of himself and process these emotions, and the feeling of loss. What I think is happening is that the son has lost himself in the relationship, and he's the only one who can choose to recover that, if he ever does. Giving the quiet space allows the son to feel that void, if he's going to.
Whatever happens- please don't cut off your son on legal papers in any way. As Pook said- it's for now, maybe not forever. Words hurt, feelings can change. The son will always be his son, whether or not the son can recognize it from the situation he's in. Your H can.