I would recommend that you take a look at the book, "Mood Cure" by Julia Ross. While it is not specifically for head injured people, she is a therapist who uses amino acids and diet to help relieve anxiety.
I have only started to read the book, but I did hear her speak online. She has worked as a psychotherapist for many years, using natural therapies to help her patients.
I hope you find something that helps.
Shelly