EL -" Once a horse person, always a horse person" - any horse in your life now? Arabians are most interesting horses. Too smart for me! There was this beautiful Arabian mare at my barn for a while who was "crazy" and I loved her to distraction. I didn't think she was crazy, she was just kind of anxious and seriously ADD. They let me work with her a bit, and as long as she had something to FOCUS on (like where she had to put her feet because you had left about thirty rails randomly strewn around the arena) she was fine. If you just wanted her to kind of blop along - disaster. But then her owners decided to move her and... well you know how that goes. I'm glad you and your boy had each other for then, and that you carry him in your heart.
I used to get "sucked into" my patterns (hurt child, issues, whatever) when H got triggered and YIKES. No broken dishes, but ear protectors and flak jackets would have been a good idea. Then, one evil weekend, he played out his parents divorce (when he was five) first doing his mom, then his dad. And after that when he would get into an episode I would just repeat to myself "I will not talk to dead people." It kept me out of my "stuff". I don't have to struggle so much against getting sucked in now, so can occasionally have a thought that is actually helpful! We communicate very well when he is not triggered tho.
Two thoughts: One: If he can't "fix it" and that is his (or one of his) main modes of relating, what else does he have to offer you (from his perspective)? Two: Is he willing to "fix" himself, or just others? PW's cautionary tale is a good one, the mirroring thing is - you are right - the only way to know for sure if you've been heard correctly - AND some people find it intolerable, or just flat out don't want to do it. So it is worth explaining it and checking first, and then... cross fingers and hope for the best!
And PW is so right - saving up conflict is just piling up gunpowder - after a certain point it just goes BOOM. Better to handle it one or two grains at a time when it comes up.