Failure is one of a teen's greatest fears. Teens think about failing at school, failing in relationships, failing in sports and extracurricular activities, and failing at life. For a teen, not being successful at school and with friends means that he is a loser. And this is his worst nightmare.
Who among us has not experienced failure? I don't know one person who has managed to succeed at everything he has undertaken. Not succeeding at a task can actually be a positive growth experience, although all of us hate to hear that phrase at the time. Think about some event in your own life that did not work in the way you had wished. Then think about what you learned from that experience. As a parent, helping your teen learn that experiences that do not go the way you want can still be experiences that have value.
Discuss your own experiences with your teen. He may be more open to talking about his fear of failure once he realizes that everyone struggles with this at some point or the other. But also remember that many times your teen just needs you to listen and reassure him.