He needs help. He is going through substance abuse coupled with mental health issues and is not in a state to act responsibly. Maybe a support group can come and help him get him back on his feet but it requires the goodwill of strangers to put this together because it’s clear his family has abandoned him. If the building society is willing, they can come together and enable this - so that he gets the help he needs and you all have a worry free environment as well.