A few references:
High school requirements.
https://www.cde.ca.gov/ci/gs/hs/cefhsgradreq.asp

DOE guidance on math acceleration.
https://www.cde.ca.gov/re/cc/documents/jul14item13slides.pdf

Note that it is not that schools are not allowed to advance middle schoolers beyond algebra I, but that they must have at least two years of math in high school (three for UC entrance). Those two years must include at least one course at the algebra I level or higher. I expect the school is concerned that they will "run out of" math courses for him before graduation (but that's where dual enrollment comes in).

Also, depending on how the HS and MS schedules are put together, private transport from one campus to the other may cause him to fall below state-mandated time-on-learning, which might be another obstacle to appropriate placement, in the school's view.

...pronounced like the long vowel and first letter of the alphabet...