Can he finish all high school grad requirements and graduate early? Of course, this could impact selective college admissions because he may not have 4 full years of language etc. Just putting it out there as an option.
Will the district pay for online accredited college math courses? Could try edx.org for free courses. Or look through “art of problem solving” website - may find someone there willing to be a paid tutor for real analysis or abstract algebra, which he can study on his own. At this point, he should start the theoretical math courses like analysis or abstract algebra anyway. And these classes are largely self-motivated, even with a real live professor.