There are lots of accredited online options (Johns Hopkins CTY, for example). Community Colleges tend to have a large offering of math courses (statistics, linear algebra, Calc 2, intermediate calculus, etc.). Do students from the local high school take courses at the college in your town? Dual enrollment is common, and students from the high school already take courses there, all you have to say is, "We can address this question when he's in tenth grade; dual enrollment seems to be the most obvious option." Saying this doesn't mean you're committed; it just means that you've offered an easy solution if they tell you he'll run out of math classes.
