Hmmm, the thought just came to my mind as to what his current circumstances are then. I take it that he lives at home still. Why? What responsibilities does he have (he should be paying rent). What expectations are placed upon him?

Is his behaviour a pattern? Has he been held accountable for his behaviour or has it been ignored and left undealt with since his youth? Has he been inadvertently enabled? This may not be a case then of a mental health issue at all.

The solution I believe is to get him out on his own. What is preventing him from doing so? If not, what needs to happen then is a laying down of the house rules and expectations - and consequences established for failing to meet them.