Let him be responsible, but assist as he will let you.
He wants to be responsible for his grades/school work, let him.
You can ask him about anything that needs to be done, get his answer, and find out it is wrong (no done) two days later.
Since it wasn't done, his eligibility to play should be compromised due to natural consequences. He still has to practice, but maybe miss the next game.
You can try to assist by lookin gin his planner and adding a few things on your own.
Lunch date with you on a Saurday for not missing any homework assignments that week.
Movie purchase on Friday for notes in his planner that are legible and homework completed. (this way you can read what he is suppose to do and not have to complain that you can't read his writing.
Special breakfast one week.
You can tell him that these things will be added to his planner. Now his planner is a tool you both use.
You can even write good job or nice game or other positive things for him.
If you work the planner as a team thing, it doesn't look so much like nagging.
By the way
I have a son in the 8th grade also.
His planner is similar. :)
I also have and overachiever for a daughter.
this is about your son. Renmeber to work with him as it works for the two of you, not how it worked with a different sibling. We all have our own way of thinking, remembering, and processing.