I used to be a guardian ad litem for numerous foster children, so I share your struggle with watching a biological parent return to a child's life and bring all sorts of instability once again. My encouragement to you is to just keep being the stable presence in his life - you know that his mother is going to possibly self-destruct once again, and that he will need you to pick up the pieces of his broken heart, as you started doing when he was 9. One day I believe when he is getting married and starting his own family, he will look to YOU as the one who got him through all of this.
From a legal standpoint, yes, it might help to get a counselor or therapist to evaluate him. If he is a ward of the state, let his social worker know that you feel his day-to-day life and emotions are being impacted by this interaction with his biological mother, and have the state order the therapist's evaluation.
Bottomline is, you can't stop him from seeing his biological mother in the short run, but you can be there for him in the long run. That is a unique role you play in his life -- many blessings on you for your love and concern for him, and hang in there!
