A professional stunt man from Granada Hills was convicted of sexually assaulting an emotionally disturbed 11-year-old boy assigned to be his "little brother" by Big Brothers of Greater Los Angeles. Mark D. Yerkes, 28, stood with his head bowed as the verdict by a San Fernando Superior Court jury was read. Friends and relatives who came to court with Yerkes broke into tears when they heard the verdict. Jurors deliberated less than three days. Yerkes faces 20 years in state prison when he appears for sentencing Feb. 28.