I wouldn't let him flounder a year
HS kids who are outstanding in their sport generally come to the attention of college coaches at the beginning of their Jr year.
California School Attendance Review board wrote:
California compulsory education law requires everyone between the ages of six and eighteen years of age to attend school, except sixteen- and seventeen-year-olds who have graduated from high school or passed the California High School Proficiency Exam (CHSPE) and obtained parental permission to leave. Some students, however, violate compulsory education laws and have a pattern of unexcused absences. Although truancy and excessive absenteeism are not new problems, they cause costly, long-term problems for the students, school, and the community.