In time your son will realise who he can actually trust to be gentle and kind, and that no-one feels safe around abusers, because when they don't play along, they are next.
This crucial.
Your DS beneath is “acting out” persona is on high alert, hyped up and distressed because he will be sub consciously scared of his DFs anger and rage and manipulation which he is exposed to day in day out and as PP have said deep down knows it will be him next.
See your son as radicalised foot soldier of a rabid tyrant - he lives in a state of hyped emotions and fear. His body is flooded with adrenaline and cortisol - he is v stressed and is being triggered and stoked up all the time. He doesn’t love this psycho.
That’s why your foil of steady emotions, soothing, gentle, kindness, love, joy and your calm and peaceful home are critical to show him and draw him to this safe place. All of the oxytocin, dopamine, serotonin, endorphins that you can rustle up for him will get you what you need.
I remember earlier in the thread you said that he was ignored at your xHs - this is important as if you give him all of the positive attention he will be attracted to this.
How is your other DS and your fiancé? What are their RS like with your DS? These are also important, independent RS that provide supportive glue to settle and ground your DS.
I would also have some reassuring but broad statements ready on rinse and repeat for any negative statements - that acknowledge his stress but also close down the conversation so that you don’t get drawn into escalating arguments / explanations - so keep it vague moving away from the actual specific content. So maybe things like:
That must be a worry for you. However:
that’s not how it is
that’s not how I see it
no need for you to worry about that
we will all be fine because we are kind, loving and helpful to each other
I won’t let that happen to you
And get him to express his feelings rather than his thoughts - so how are you feeling right now? Are you scared? Confused?
Anger is often a secondary emotion that is triggered in defence or under stress. Beneath the superficial anger is often fear and hurt. Tap into that behind the anger if you can so that you can soothe and reassure his deep fear and hurt rather than react to or be wounded by his superficial anger.
You are doing great. Take your wins hour by hour.