If you are planning for him to audit a Lab class, such as Chemistry, there is also the issue of safety. With Physics, there are Physics classes that require Calculus and Physics classes that don't.
Also worth figuring out if there is a local Community College within driving distance as CC's often offer High School Level classes, but twice as fast with perhaps less red tape! SAT scores in hand tend to pave the way here.
High schools vary so much, but in some he could take AP level classes for credit when he gets to high school agewise, using the base knowledge he got while auditing. Also Independent Studies might be an option.
Best Wishes,
Grinity
