Happily there is likely nothing “wrong” with him, for a variety of reasons he has been able to follow an instinct to withdraw and not be forced out of his comfort zone. I’m similar - if I can avoid people I will, but I know it’s very bad for me (depression, awful self esteem and feelings of hopelessness, spiralling negative and intrusive thoughts and anxiety, feel panicky in new situations, can ultimately feel so cut off and isolated that I’ve felt suicidal) so I force myself to be sociable so that I don’t lose the “hard coating” I’ve picked up by repeatedly exposing myself to social contact.
My older db was similar - naturally reclusive and hid in his alternate game universe to which he was totally addicted.
Gaming is a terrible addiction. My mum kicked my brother’s ass until he got a job. Lots of shouting and unpleasantness but in the end he launched and has been mostly fine ever since (though covid also was awful for him).
So here I think you have to be very kind but firm as doing him absolutely no favours letting him wallow.
You charge him rent and bills/electricity. You give him a long list of chores - as physical as possible (all the laundry, clean car, windows, do garden, change beds) AND include him doing the grocery shopping, make him pay for it from UC. Do not buy or cook him any food or pay for anything for him. No lifts unless he offers something in exchange - make him walk and get fresh air (even if it means he’s shopping at the grocery store every day at least he is out.
You treat him like a tenant. If he makes a mess and doesn’t tidy up you tell him he’s on a month’s warning then he’s out. No drama just factual.
I assume he cannot afford aa car
At weekends you take visit to see relatives - he needs to see people outside the home. Tell him if he wants to live with you, he has to buck up and be useful. Ideally he needs life to become less comfortable with more activity outside the home of any kind.