Good advice above. All forms of therapy are rather therapist-dependent. Sometimes it takes a few tries to get the right fit.
My usual sequence would be to have a psychologist evaluate first, in case the anxiety is a symptom of something else--you mention that he is having to write a lot at school, a disfavored activity. If writing is challenging for him on some level (handwriting, idea-generation, organization, connecting with teacher-selected topics, etc.), the CBT will not change the level of challenge presented by the stimulus for his anxiety, which would limit its effectiveness. A psychologist is more likely to investigate those kinds of triggers, and possibly provide you with some educational recommendations, or neuropsych-based strategies.
If the psychologist does not find something specific, then I would pursue a psychiatric consult, but make sure it's someone experienced with children. A really good child/adolescent psychiatrist can be very helpful, not only with meds, but with strategies.
