Hi @Lotty. I can identify with a few things with your son. I've also had severe social anxiety my whole life so I know how much of a struggle that can be in so many ways! To be honest, I didn't move out of home until I was 44, and that was only because I met my now partner and we moved in together. I'm not sure what else would have made me move out. It's not that I didn't want to, it was more that I had no where to go. I couldn't afford to live somewhere by myself, and I didn't know anyone I could move in with due to not knowing many people because I didn't have any strong connections with anyone. And the thought of moving in with someone I didn't know was far too much for me. I was good and pretending nothing was wrong. It was far easier just to let people think I was lazy, unambitious and was just a person who preferred to being alone than admit the truth... that interacting with people, specially those I didn't know, terrified me to death and I would do anything to avoid it.
So I wonder if you son has similar issues. From my point of view, it would explain why he gets angry when you bring it up. It would force him to admit those feelings of failure he may have due to his social anxiety which can be very traumatizing. As I said, this is just based on my own experiences with social anxiety.
I also was put on a DSP this year due to being diagnosed with stage 3 prostate cancer a couple of years ago. I'm not sure how I would have gotten through without my partner, but even then I probably wasn't the most fun and joyful person to be around some of the time... probably still aren't. It was so much easier for a lot of time to just hide from the world and do whatever I could to keep my mind occupied to keep my mind off my health issues. I still have this same problem, but it took a long time and I still have a lot of those days.
Obviously I don't know your son other that what you have written, but since I did notice some similarities, I thought I'd share my own experiences in case you found any of it relevant and helpful. I think it's probably in your best interest, and in his, to start setting some boundaries with him, because the current situation clearly isn't helping him in the longer term. He may know that, but feel like he doesn't know how much of a longer term there is, which was something I really struggled with at my worst. Doing anything can feel pointless a lot of the time.
Is he seeing anyone for his own mental health? I know when I was diagnosed, and the same with a lot of people I have talked to, that you get told of everything to expect from treatment and the related side effects, but no one prepares you for the mental strain and you're sort of left to yourself to discover that and seek help. My partner works with clients dealing with the NDIS, and comments so much that mental health care is not officially seen as part of the support required for people dealing with life changing health issues. Which is just wrong, because it's a massive part of it! What helped me most was just finding people to talk about how I felt. Places like here, my local men's mental health group, and even just some of the nurses was incredibly helpful for me during the worst times. At the start, I just wanted to hide but it eventually got so unbearable that I 'allowed myself' by a nurse to talk to someone, which led to talking to more people and it really did help so much to get me through those very dark days, and into the 'more gray' days now.
So like I said, some boundaries would be helpful for everyone involved, but maybe start small... like him having dinner with you and your partner on the weekends to get to know him better. I know I did the same thing with people I didn't know very well, but I didn't like myself for doing it... I just needed someone to push me so I could use the excuse I was doing it to please them rather than I really wanted to. Like I said, the social anxiety, really did mess with me quite badly. And everyone deals with it differently
So talk to him, try to understand why he feels the ways he does. He may be completely different from me. But try to let him know the benefits of living by himself and that you care and are just trying to understand. Let him realize that you know he has some serious things going on and that you just want to try to help and understand better because you don't like seeing him this way, and just want him to be happier.
Good luck Lotty and let us know how you get on, and don't forget to look after yourself as well.