You don't indicate what has changed in your son's life this year -- a family move, a new baby, a separation or divorce, a death in the family -- all of these are events that can cause a child to become more sensitive. Your son may also feel intimidated by his teacher or some of the other children.
Talk with the school counselor. She may be able to give your son some individual time or include him in a small group. If you decide to seek additional help for him outside the school, the counselor or your pediatrician can refer you to a therapist in your community.