I encourage you to make an appointment with his school counselor right away. Find out what the school sees or has learned about your son's achievement, study skills, and abilities. If he has not been tested, insist on his being tested now.
Work with his counselor and teachers on an educational plan. Your son may need a study-skills class or resource room, tutoring, special reading class, or an alternative program. Most of all, he needs support and patience. If the school has groups for children of divorce, ask that he be included.
For yourself and for him, enroll in a parenting skills class where you can get support and help at this most crucial time of his development.