Talk with the school counselor. She may be able to give your son some individual time to work on his test anxiety. There may be a college or university near you that also offers this type of training for students.
There are many simple relaxation exercises that your son can be taught to help reduce his anxiety level in testing situations. He can learn to tighten and then relax specific muscle groups in his body; close his eyes and imagine that he is in a calm and peaceful place; use positive imagery to imagine finishing the test successfully and making a good grade on it. Check your public library or a bookstore for a book on stress reduction and relaxation for other exercises your son can practice.