Your H may or may not break through his shell, but one thing’s for certain. You won’t break through it.
Another thing you should realize is that he may not be hurtin’ as bad as it appears. I’ve had a tough childhood too. It still hurts me sometimes, but when I’m hurtin’ the most, something my father once told me always pops into my head, “When your mother died, you (kids) stood back to back against the word.” Why do I always think of this? Because, though I don’t brag about it, I’m proud of the way I handled diversity. Amidst my pain, there was (is) pride. Sounds kinda warped, but it was my path to healing.
Maybe it’s your H’s path too. Maybe he has issues, but I don’t think you should sweat them. He’s dealing with them. All you have to do is to support him when he allows you to, and to let him deal with them by himself when he doesn’t. (which is also a form of support)
I also wonder about the things that you say you’re coming to realize that you can’t change. Has he always been that way? I know he’s always had an elevated sense of responsibility. I can guarantee that this aspect will never change. It’s a source of private pride, as I stated above.
But, if he was more demonstrative (for example) earlier in your R, then I think he can “change” back. It’s not an ingrained value for him to be reserved. Of course, you can’t force him to be more demonstrative, but it could still happen.