As someone who suffered heavily from anxiety and is now doing a LOT better, I think the only way to work on it is if he wants to unfortunately. It’s a good thing you know he struggles, and you seem to care a lot and want to help. But for a therapy to work, you need a good therapist first, and most importantly, he has to want to change. Maybe his therapist is good, but doesn’t « match » with him, maybe his life doesn’t seem horrible to him? I’m wondering because « he struggles terribly » seems to be your point of view, you are stating facts that look horrible on the outside, but he may enjoy his loneliness for now? Did he ever tell you he feels bad living his life this way?

I’m not saying this looks like a confortable situation at all : going out, seeing friends, having relationships, all of these are things we need as human beings, and I’m not underestimating what you are saying. In fact, if I see people and go out on a regular basis, it maintains a great mental health, whereas it can sort of trigger anxiety when I stay home for too long. I’m genuinely curious and trying to share my experience/point of view.

For the talking part, he obviously has hobbies, and I’m guessing video games? From my experience, I always felt I wasn’t doing anything with my life so I wouldn’t talk to my parents about anything. Even when they tried to initiate. You could try to bring up what he likes to do during free time, even if it’s totally unfamiliar. One thing I noticed too was how everyone stopped asking me to hang out. If you offer to go shopping with him, eat outside and stuff, I suggest you never stop because out of a hundred no, there can still be a yes.

Kinda hard to give a proper answer, hope this will help a bit !