he could be a former substance abuser who's alienated everyone in his life.
he could be a schizophrenic man whose hallucinations have burnt out but who's left with the cognitive issues that plague their later lives.
he could be coherent and bright but still have mental retardation.
he could have some combo.
the secret tool that psychiatrists use is to ask, "hey, listen, I was wondering why you're homeless. do you have a theory?"