oh that poor boy...he must be so confused and angry. Have you considered suggesting to the mother that he get some counselling? Even a weekly or a bi-monthly session might help...There was a period (during which my XAH drank A LOT) where my stepson had a LOT of anger and unresolved emotions; I had the social worker I was seeing call his school counsellor. The counsellor brought him in on a few occasions to check in with him;it seemed to help a great deal.
As for you, you're doing SO well at handling this highly emotional situation and not letting yourself get sucked into discussions that will lead absolutely nowhere. Good job!