Talk with the principal about your son's class setting last year and request that extra care be given to his class placement for this year. Ask that he be assigned to a teacher who will be caring and understanding of his concerns. Make sure that your son meets his teacher before school starts and when you are with him so that he will know that this is a safe person who has your approval.
Try not to put any pressure on your son about the placement testing. Be very casual about it, explaining that the psychologist just wants to see what your son knows. Putting too much emphasis on the importance of the tests may make him anxious. The psychologist can also help you by talking with your son about what to expect next year at his new school.