Hi,
My wife is a family therapist and our son who is now 20 has ADD and depression/anxiety and has been on and off medication. Our son was never as defiant as you describe, but I can relate to your feeling of being overwhelmed and at your wit’s end. You mentioned your son is in therapy. I recommend that, in addition to his individual therapy, you also do family therapy. Family therapists, especially MFTs, approach therapy from a systems perspective and can be much more helpful in dealing with defiant children, because they hear both sides and can help both sides with coping better.