Hello N.,
You have received many excellent comments.
To reiterate what several have said, you need to know why your son was retained. Is he missing basic skills- reading, writing, math, study skills? Does he have social issues? Does he have an IEP? Have you attended the parent / student conferences. With school starting or having started, make an appointment with the guidance counselor, and case manager (if he has one).
If your son does not have an IEP, and you feel and the school feels he is lacking some basic skills- education or social, ask for a special meeting to see if your son qualifies for special education services or for a 504 plan. Be proactive, and the advocate that your son needs.
As a high school teacher if a child is not academically or socially ready for high school, retention, tutoring in the 8th grade is a good idea. Did your son complete summer school to make up for any classes he did not pass while in 8th grade?
As several have said high school has more expectations than the middle school/ junior high. Once a student is in high school they need to earn credits / or meet competency on common assessments/ state exams in order to graduate from high school. Look at the long term needs of your son, and how he will accomplish them.
Good luck. Also as a parent know what your son is doing, check the district web site and see what his daily grades are, and also what his homework and long term assignments are. Monitor his work, be sure he completes and passes in all work. You may have to ask that the teacher initial his agenda book each day to verify assignments, and then you may initial the page also so the school knows you are aware of his work, and when it is due.
Another thought is some schools have an alternative education program, both for middle school and high school. This might be another possibility for your son, if your school system has an alternative education program.
Be proactive. Good luck to you and your son for a productive, successful school year.