I'm curious how you come to the conclusion that the CP "let" him fail. Was it not your son's responsibility to do the work? To go to class? To pay attention to the teachers? At what point does it stop being the responsibility of the parent to push the child and start being the child's responsibility to actually perform?
Exactly. And the child has two parents here, albeit divorced. To be exercising eow and one day a week, OP must be near enough to be involved. It's easy to point fingers, but if OP wants to do that, the blame must be shared. Both parents failed this child, especially as it sounds like an issue that was in play for quite a while. If OP has been paying support for 9 years, this is a problem that has been years in the making. If one method doesn't work, another should be tried. Not all kids have the same aptitudes or interests, nor is every child destined to become an Einstein.
At the end of the day, everything about support will depend on what the order states about when CS ends. More likely than not, it is 18 OR when the child graduates or 19, whichever is first. If he's actively enrolled in an online school, the court will likely accept that for purposes of continuing support until 19. You're not going to get out of it because the other parent "failed", in your opinion.