Regardless of him being treated for ADHD/GAD, he would still have to take responsibility for his aggression/low frustration tolerance. You would want to get his side of the story (did he get so mad because this other child bullies him, did he think his friends will think less of him if he did not defens himself, did he try to tell an adult what was going on, etc) Then you would speak to him about anger/aggression and how people can hurt one another with their words (like his peer did by calling him a name, and how it does not make it any better to retaliate against him)
You can let him know that people make mistakes (his peer did so and your son did so in his reply) and that it is more important to know what is the right response should someone aggravates him in the future. Then you and him can talk about ways that he could have handled it differently and making sure that he learns and applies anger management techniques. Teach him that at times it is best to ignore the negative behavior. You can think of some consequence that may be fair (this is after the fact) Perhaps have him write some ways that he had used in the past to help him with his frustration and post it on his wall at home.
Yes we have been through all of that with him before, and the assistant principle whom called me said that he has gone through all of that with him today as well. The principle did not think that he had gotten through to him. I think he is probably correct. This is the first time that he has acted out physically, but, unfortunately, not the first time that he has taken his anger out on others. He says that he doesn't know how to stop himself. I know that the transition from the Thanksgiving holiday was tough for him, but I am really at a loss here.
His behavior won't change over night. If low frustration tolerance is an issue for him, then he can be enrolled in anger management classes for children either if the school counselor offers that or through a community mental health agency. There would be no point of talking to him on and on about his past mistakes. You've done it and the principle had done so. The next thing would be to act- come up with a plan of what is expected of him, what the consequences will be in the future and make sure that these rules you set for him are enforced by others in the family- grandparents, your partner, etc. Speak to the school counselor to see if she has any resources for anger management for children. He is simply reacting to his triggers and he can learn to practice self control. It may take some time, but with enforcement and encouragement, it can be done. He has heard what you've said even if he is not taking it seriously. Once he starts getting monitored and consequnted for his behavior, he may connect his own behavior to the undesirable outcomes for himself.
Indigo Ocean Dreams: 4 Children's Stories Designed to Decrease Stress, Anger and Anxiety while Increasing Self-Esteem and Self-Awareness by Lori Lite (Audio CD - Oct 17, 2006)
Parenting a Child Who Has Intense Emotions: Dialectical Behavior Therapy Skills to Help Your Child Regulate Emotional Outbursts and Aggressive Behaviors - Paperback (Nov. 2009) by Pat Harvey and Jeanine A. Penzo