That said, I do encourage you to have a family meeting in which you emphasize that you indeed are getting off his back and letting him take charge. Should he choose to not accept responsibility and if he fails any or all subjects, then he will repeat eighth grade, go to summer school, not enroll in football - whatever you decide. You could also throw in a reward, such as, "If you bring your grades up to a C+, we will go to _______ or you will get to ________."
The most important thing is for you to follow through and to do it without anger or revenge. If he should fail, impose the consequences and assure him that you know he can do better next year. Then drop it. Go forward with the expectation that he has learned and will do better.