The child's brain has been permanently damaged by the abuse he suffered. Arguably, perhaps he can be rehabilitated to an extent - maybe a considerably extent. Hopefully so, but the main thing is, are you willing to sacrifice your peace of mind, the health of your marriage, the safety of your pets, your reputation, your financial health - if he does something horrific to get you sued - to offer this boy whatever modicum of 'help' you can?
The nature of his brain damage is that he is unable to function normally in a family setting. At this point he needs institutionalization to keep him and others safe. You can help him by continuing to remain a presence in his life, but from a safe distance.
Just my opinion......most authorities would say that a kid's behavior is pretty well hard wired after the age of 3 or so.
