How can I help my brother?

Me (33, f) and my brother (35, m) were really close growing up. Both of us were very anxious kids at a time when most parents didn't even know anxiety was more than just a feeling you got sometimes, so by the time we were 12-14 we were more or less allowed to isolate and avoid social interactions. Mom worked 12+ hours a day and Dad was no longer around, so we raised ourselves from this point on.

No hate to my parents here. They were going through a wild divorce and everything just sucked for everyone. At some point during everything that was exploding in our lives, my brother was diagnosed with social anxiety. He took a medication for maybe a few days before giving up. Mom didn't even notice.

My brother was bullied throughout school, though over time I've started to question how much of it was actual intentional bullying and how much was his own anxiety tearing itself apart like a nervous Chihuahua with allergies. Regardless, it was so intolerable for him that he stopped attending and would hide under my bed instead so my mom wouldn't notice him as she was getting ready for work. He once jumped out the window on the second story just so she wouldn't catch him.

Age 18, he goes to live with Dad for a while. He actually does well (our Dad is tough love and can have a temper but he's a good person), then comes home and regresses immediately. Worse than ever.

I stayed home for way longer than I should have in my twenties, waiting for him to 'fledge' with me. I'd talk to him about getting an apartment together if he just gets a little easy peasy job etc etc

Nope. Never happened. I moved out at 28 and as far as I know he leaves the house every 6 months if necessary. When I visit, he rarely comes out of his bedroom to say hello.

Mom is in her 60s and in poor health, the house is falling apart around them, I work an entry level position where even a one bedroom is tough, so I can't be his safety net when she dies, which I think is what he's banking on, even though he says he has it all figured out.

He won't seek diagnoses or treatment. He's currently on welfare but at this rate he also hasn't filed taxes in idk how long so the whole thing is a mess. With a diagnosis he could arguably get disability payments but I can't do any of it without his help.

Just... Need some help. Does anyone know a gentle way to approach all this that won't make him run away or lash out?