You just have to be psychological and metacognitive during the whole argument. Pin point what he’s doing. (Trying to be right, saying something to feel righteous, lying to perpetuate the argument) and don’t sit on it. I have had a bipolar alcoholic mother my whole life. The moment I break everything down and show my point, they always try to say something non related, or something else to try and justify the conversation. However, when that happens I comment on the whole situation.
Example: “why bring that up when it has nothing to do with our conversation (explain why it has nothing to do with it) then if they try to deflect, press down on your point. “Why are you trying to shift the conversation to something unrelated. Is winning the argument that important to you? Why can’t you specifically address what I said” all of those questions and points, dictate the conversation. It helps the person not get away with gaslighting and manipulation. Always stay metacognitive.
Your objective is to address the problem of not being able to communicate, not the objective of the argument. Sorry to type so long but being a psychology grad student who hopes to get a phd in familial and developmental psychology, I am very passionate about this. It takes a strong heart, but at this point forward you need to put your foot down and press the conversation to address your issues. Be mindful and listen, but never back down from your objective