M.,
It is very possible that if your son has an unidentified learning issue, that he can be promoted to 9th grade in public school. Why do you think he failed 8th grade? Was it a homework issue? An issue with how the catholic school expected him to show that he learned the matierial? If he has some kind of a processing or output issue that can be measured than you have a very good chance of getting him into 9th grade, because you have independent data that he knows the mateiral, but failed to show the catholic school in the way that they asked.
Your first step is to enroll him in public school, and appeal to them for the promotion and request that they identify any learning disablity or processing issue that may qualify him for services to make general education accessable for him.
You should be aware that retention is a very bad thing for kids. It is the most reliable predictor for dropping out of school. You should do everything in your power to see that your son stays at age-grade level. If your son has a learning issue, it is even more important that he recieve targeted instruction, intervention, and accomodations rather than another year of the same old same old which is absolutly the wrong thing for every child, from kindergarten on and has been shown again and again to be a very bad practice.
Be a squeaky wheel. Get him into 9th grade-you are holding the data that supports your case!
M.