This sounds like a case of it's better to be with someone than be alone ... although it comes at a high price for you.

For what reasons do you stay?

Do you think you could do some research into therapists that he could see, and let him know when you're booking an appointment for him? He'll have to keep his commitment to the appointments otherwise he's not being serious about anything.

Also, worthwhile contacting your internet service provider. My provider lets me set up a control level (it's actually parental controls) but if you set it to medium / low it should be able to block porn. Alternatively I'm also able to block websites with my provider.

Alternatively my internet service provider also offers free porn filtering.

I'd get in touch with yours and see what they can do for you.