Some people are simply not good at school. He can be terrible at school and barely graduate, and go on to be very successful.
Is an online school or GED a possibility? He needs to get past school that teaches what others think he needs to learn to learning and working in what interests him. The ADHD brain is motivated by interest and urgency, NOT by importance.
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism