Where to go for help

Ask about CBT, I am sure somebody else may have suggested it. You can usually self refer to the local 'IAPT' service in most areas. Just go online to your nearest NHS service. Hope he finds the help he needs - and deserves.

I don't suppose any GP has an instant cure for this. Poor lad. He must have been referred on as he's had CBT.
I assume he's on benefits? As part of this is he not supposed to attend various group type activities? That can be beneficial.
I was very struck by the Johann Hari book on depression. A lot of useful insight and info. It certainly helped me to find a different way of looking at and understanding depression. Practical help too.

And try another GP.....they are not all the same, some fantstic and others terrible.

How about contacting MIND www.mind.org.uk/information-support/ , the charity that helps and advises those with mental illness or concerned about someone with a mental illness.

You are a caring, loving auntie and he is lucky to have you in his life. Your suggestion of him visiting for a change of scenery seems like a good idea. Is it possible for you to go there to accompany him to your home? The journey itself could be a good first step to his new life, away from the confines of his room. I think baby steps for now with encouragement as he goes. It's so hard and there is only so much the GP's can/will do. You don't say what his parents' attitude or position is but maybe they have become apathetic over the years.

Oh that's such a shame.
The thing is though, that when you can't see any light at the end of the tunnel, everything seems too much effort.

I’ve suggested it MissAdventure, but he doesn’t want me to.

Could you perhaps go with to the gp?
Its a long trip, I know, but he might appreciate someone 'taking up the reins.'