Ok, here are some thoughts from a US perspective.
What I think he probably needs is a social worker. You might call social security disability offices and see if they can point you to one, or call United Way, or google for nonprofits related to whatever his issue is (mental health, learning disability). There are often subsidized apartments for people on disability, they might be able to help get him in one and away from roommates, and know what to do to keep him safe.