If it were me I would make the stipulation that you are only willing to remain with him if he engages in treatment. He does not need to be "going to therapy" to be treating himself. I have been in and out of therapy (much of it unhelpful) but still regularly, daily, engaging in active management of my symptoms and directive goals for about sixteen years. It sounds like a good first step for him would be to look more closely into anger management. The good anger management that is geared toward grown-ups and doesn't use condescending language that makes us feel stupid or give advice that is not rooted in actual science, is typically based on a 3-axis model: distress tolerance, education and behavioral modification.
Most anger management tools can be quite condescending, I found some good resources from the VA and in particular, AIMS, and the distress tolerance tools of FORNET to be the most effective. In particular "stop" and "distract." For ex I have an app on my phone that says "look at these soothing pictures!" but it's just memes of Draco Malfoy being like "wait til my THERAPIST hears about this" in blingee font. I don't know why but it makes me laugh because I'm dumb. One time the VA app told me I shouldn't bite people. I screenshotted it so now all I see is "DON'T BITE" and it makes me laugh.
Like, stupid shit. Having a plan for when he does lose his temper will be more effective than trying to manage it in the moment. The biggest issue with anger management is that once a person's heartrate and BP go above like 130 (according to Patience Mason, anyway) the ability to engage with your brain's logical and decision-making processes is completely decimated. For me, using the tools from the VA was helpful because when I get angry, my brain has a separate, intentional-conditional pathway to be as aggressive, violent and terrifying as I possibly can -> this is stuff that is also taught to military veterans (via boot camp).
So in many ways, "de-programming" this response can come with a good deal of annihilation anxiety, since "killing rage" is extremely survival-oriented. Having the ability to document the things that make me upset on a day-to-day basis was somewhat helpful, but I found it similar to trying to document my dissociative disorder: basically, I am always in a state of heightened agitation, so anything and everything will set me off and there is no rhyme or reason to it. The problem is neurophysiological, not environmental.
Another tool that has been helpful to me is to attempt to "kickstart" the logical engagement process (usually I need to rely on my phone for this, as I have it written down) where I remind myself that I am in total control of my behavior as an adult and if I lash out violently, I am responsible for that, and I have the choice to act compassionately and reasonably even when I am angry.
Learning about the aggression pathway and the dialectical component of FORNET was very useful to me because I can see in real-time how it impacts my daily living. I get angry, my brain fires a pathway that has been fired millions of times in the past since childhood (what we call appetitive aggression, which is that the more violent you are, the more violent you will be) and because I am so accustomed to that happening I don't question it or stop to engage with it or try to redirect it at all. Especially because a lot of the time, "blowing my top" or losing my temper or engaging in aggression? Is useful!
People are scared of me so they do what I want. I get what I want, I don't have to exert much effort, and I get my needs met. But obviously this is not sustainable in a reciprocal relationship. What good is it if your partner is scared of you all the time? How is that going to promote intimacy and wellbeing in the household? How are you going to raise a child who grows up to be a conscientious citizen when that child is learning the same exact aggression pathways that you have, because you are teaching them the same exact responses? (That very likely began for us, in early childhood.)
Similarly, reducing my exposure to violent content (media, TV, the types of stories I read, in general the content I consume) has helped a great deal by "de-programming" those pathways. Even things like video games, or pornography, or whatever other thing. This is a hugely intentional process that essentially requires totally adjusting how you live your life on a day-to-day basis. It's not easy and it requires the willingness to do it for yourself, because your relationships are suffering. You can't half-ass it, so if he isn't willing, it's not going to work.
Part of my plan involves making sure that those around me understand what they can and cannot do when I get angry. For one thing, they cannot engage with me. They need to leave me alone, and respect my boundaries when I physically remove myself from the situation. If that doesn't happen my capacity to control myself disintegrates and that's when the situation devolves. I cannot manage anyone else other than myself. So someone going "ughhh stop being so mad!" or "idk what to tell you!" or even "omg I'm so sorry" (or, "I understand," or literally anything).
My responses are not rational, but yours are. I'm going to react irrationally and probably yell back something stupid like YOU COULD NEVER UNDERSTAND!!! and throw something at you. This is unfortunately not quite where I should be (I am responsible for my own behavior in all circumstances, or at least I should be) but having my carer understand this has reduced our "conflict zone" by a significant margin. I know when I'm getting frustrated and I will just leave, && she does not follow me. Or I will say "I can't talk right now" and she leaves me alone.