I'm not sure he isn't being treated? "struggles with emotional dysregulation" doesn't really sound to me like the wording you'd use for someone who'd never been to therapy about it. (You may have been to just enough therapy to learn the wording to make excuses for it, but---)

But a lot of what's in this letter is confusing to me. The boys are still living at home; they come to the mother to say they felt their "childhoods" were traumatic and they may cut contact "once they are grown". Does that mean they all feel like the husband has been improvement with some kind of treatment and the abuse is no longer happening? Does that mean LW is eliding the fact that it's still a traumatic household? Does that mean the teens are now old enough to just spend all their time at friends' houses to avoid it, and that's why the subject came up?

LW isn't saying anything about what's happening in the household *now* other than this one conversation. LW, what is happening in the household *now*? That's what you can actually work on right now; you are kind of lucky in that your kids did feel safe enough to come to you now instead of *after* they were grown and cut contact and you lost this last chance to prioritize their safety at home.