Here's a thought - look for placement tests available online for homeschooling curricula, for 7th and 8th grade math. I think I have one for Saxon and one for Singapore; each test is about two sides of a page. Print them out and let him do those in the down time after finishing the standardized tests. If he knows it all, great; if he misses a couple concepts, let him learn those; if it's all new, it's likely he would learn something in the regular class next year. If you have access to the actual textbook the classes would use, they often include unit tests or reviews that he could go over in the same way. It feels a bit like test prep, but if he can learn everything he needs to learn in less than six hours, it wouldn't be worth sitting through the classes.
