I agree with Pisces, that the work to figure out this problem has to be on his own.
I think you could still show him friendly concern, and ask him how he's doing with it the next time you sit down and have a conversation, though. Can you keep it light "how's your health?" and your own fears for him out of the conversation (i.e. nagging, or whatever he felt you did before)?
But despite what you talk to him about, I think it's important to understand where he's at for your own sake. Not so you worry about him, but so you can deal with the place that he's in and DB to that place. From your earlier posts I couldn't quite get where his head was at. Now I think it's coming clearer. You guys have been through a lot. I don't see him so much as disengaged from you anymore as seriously trying to keep his head above water with his own issues--I think that's a big difference.
Dark and dim, or other techniques may not work for you & me the way it should because our H's are in this I-don't-know-what-to-call-it space. I think P has had the right approach being a super-supportive friend. I look at her and I'm like, "I've seen the promised land!" Now, we just gotta figure out how to get there...
Pisces, do you remember how you FIRST made the transition from H blaming you and running away to your first stab at real friendship and QT? I know you built it with acceptance and validation from there, but how did you cross that first gulf?
It is in the shelter of each other that people live.--Irish proverb