similar to yours & we wanted him to learn that 80% of grades is simply turning in assignments. To get a phone he had to have no missing assignments in any class for 2 quarters. He never had such a high GPA before or after that half year. His grade might have dropped a bit after that but at least he knew it was in him & he wasn’t stupid.
Your son may not believe he has it in him. Maybe he does have something getting in the way but until he cares enough to really test his abilities, he may never know. He also may need to learn how to learn. Gaining those skills may require some direct instruction.