Great work, Aaron.
The fact that he was able to express that something made him angry is a great sign. I would explore what about your relationship led to that feeling and allow him to express directly to you anything that he can and is important about that. It will model for him that he can allow anger and express it in a safe and healthy way (with you).
Then you can also explore what might be underlying this; what in his life has led to him being angry. He may have been told that he was safe when he really wasn’t by people who had power over him. There was probably something like that that triggered his reaction. Then you can do emotional processing work (EAET) with him on that situation from the past.
It’s also great that you helped him to tolerate being anxious without fearing it. That is the PRT approach to anxiety (and pain).