NEED HELP-How to help/support my sweet half

Hi all
My bf has been addicted to masturbation for years. (I think you call it PMO)...
We've been together for 3 years and he told me about his problem right when we met (it started when he was late teenager now he's almost in his 30's). He says that his behavior hasn't got any better with time.. I know that it comes from a dark place of depression and anxiety. He sees a therapist once a week and fills his time with work and gym but it's a vicious circle and in just a glimpse, everything can go to hell: he isolates himself from the world and caves in. If he didn't work I really think that he could just stay home and fap all day.
I would like to have hints on what to do in order to help or what behavior should I have ? Should I support.. but without encouraging the behavior nor making him feel that is ok because it's really not. Be more assertive ? Talk about this more often (it's never a good time to "talk about it"..)
My fear is that it could easily be overwhelming for him to have to expose this to me on a daily basis and the opposite effect could happen as he could close down to be too...
We love each other and we've been on a LDR for 3 years now. It's not easy but manage to find balance but I feel that his issue is also mine now and I have to do something about it.
Please help, I'm pretty desperate, I love him deeply and truly, but this gets to me and makes me really sad.