Hey Jess,
I think your H sounds like a very sweet guy who's toting a big barrel of pain. However, I think he's also extremely lucky. He has your love and loyalty and most of all, he has the impulse to heal...he's open to the idea of therapy.
Tell him a good therapist will understand more than he ever thought possible, and that he's making a wonderful, wonderful choice if he does this.
It's the rest of his life in the balance.
And with a good T's support and insight he too can build the strength to build boundaries.
In fact, Google "boundaries in relationships" -- that is often a major eye-opener.
And Jess, you too are a sweet and loving person...to bring his troubles here.
But once you give him the support and encouragement to go get help, your support will have to take the form of supporting him in HIS work, but not doing it for him. (Though you could go along to he counselor for a few sessions if he'd like you to...that can be very helpful in some cases.)
Kudos to you for your kindness,
Hopalong