Anon, here's my question for you: Does HE want to fix this problem?
There's really nothing you can do to change him or to help him address this problem unless he wants to. From what you describe, he has problems that he should be in therapy about that stem from his depression, drug abuse and psychological issues. That's a LOT to deal with in and of itself. The fact that he masturbates and uses pornography while telling you he's not interested in sex says a lot about how he sees you, and how he sees sex. It sounds like he doesn't see the situation as a problem, while you do. As long as that basic inequity remains, I think the situation will probably stay exactly as it is.
You sound intelligent, thoughtful and compassionate, but you have to understand that when it comes to being able to change another person, there's not much we can do unless THEY want to change.