My daughter is in treatment and has lived in the wake of her brother's chaos for 6 years. What I have learned, above everything, is that no matter what the subject is, it is important to just listen and validate his feelings and experiences. Her dad (my ex) used to say to her ALL the time, "Oh, stop being so dramatic!" .... thinking that she was just being a typical 13 yr old 'drama queen'. Now we are realizing that she was screaming for help and for her distress to be recognized. But because she was a good student and didn't get into trouble, we were able to focus the rest of our energy (about 98% of it) on trying to control the chaos our son was producing. She was not being heard. She is suffering now because of that neglect (as well as some other family dynamic issues). If you are not familiar with how to "validate" feelings, I'm sure your therapist could help a lot and, of course, there are many online sources of information.
We didn't recognize our daughters' needs until she had a complete emotional breakdown at the age of 16. We almost lost her for good.
Please keep the focus on your son and keep him in therapy for a long time. He has many new skills to learn. All of you do.
From one suffering mom to another -- (((Hugs)))