My heart goes out to both of you, you're really up against it. Poor lad and poor you.
I get completely that it's important to him it'll give him distraction, purpose, comfort and possibly a type of self medication almost from spiralling thoughts.
He's had no control over things that have happened to him so perhaps this is one tangible thing that he feels he can "own" or have control over and that gives him happiness rather that pain. For that to turn into something so difficult for you both is such a shame but I completely understand where you're coming from. You're bound to want to protect him and ensure you have control but maybe what he's doing is soothing him and limiting his use to such an extent is upping the ante and making him want that control more. Please, please don't think I'm criticising, I promise you I'm not, you're in a heartbreaking situation but what is happening isn't working for either of you so maybe turning things on their head might be worth considering.
You mentioned that you've never had any problems with him accessing inappropriate stuff. Is he in a position to come to an agreement where he can actually increase his use but only in certain circumstances e.g. When you're in the same room, as a reward or whatever works for you?
I know this is coming at it from the complete opposite direction but of it's effectively a comfort item/blanket for him, then I think I'd try to remove the battle and see if there's a way of coming together with him, finding out what he wants and upping safe usage. Maybe there's a way the therapist could help work something out.
I've worked with a lot of survivors of sa and their families. You are doing an amazing job but please make sure you get support to deal with how you feel too, he's not the only one whose trust has been broken and abused, so has yours. Take care of yourself too.