...it was actually all algebra concepts (factoring polynomials, solving for x, simplifying expressions, negative exponents, etc.). He has had no real prealgebra instruction.
DH and I are now ambivalent about which path he should take. We have full confidence that he would do well in algebra because he learns so quickly and math really is his strong suit. He just seems to "get" math. If we did give him some exposure to it, we feel like he would pick it up easily. We are in no hurry to move him through the curriculum (and finishing the school's curriculum two years early would bring its own challenges), but on the other hand he has never had to learn new concepts in math class or actually work to learn something new.
My understanding of pre-algebra is that it's just algebra-lite rather than a discrete subject that's different from algebra. I've looked at a few pre-algebra books (including the new Saxon books, which BTW, I find to be scattered and not too great), and they confirm this idea.
The problem with pre-algebra and HG+ kids is that it draws algebra out over two years, and year 2 repeats a lot of year 1.
I suggest that you consider homeschooling him in algebra. He's very young, and I wouldn't worry about what might happen six years from now. You have no idea how the school will change or if you'll be sending him to that school or if there will be more online or community college options...etc.
If you do homeschool, I recommend either the 2000 edition of
Algebra: Structure and Method or a version of that book dating back to the 1960s. The first editions of these books were written by Mary Dolciani. The books are focused and offer a variety of problems ranging in difficulty from easy (oral exercises) to challenging (C-level exercises). You might also want to pick up a teacher's edition for yourself. If you find a California TE, it's the same as the other TEs. It just has some extra text about California standards at the beginning.
If they insist that you use an accredited online or other system, you might consider something that will let him just take the tests online. The Uni. of Missouri ["Mizzou"] Online High School is cheap, provides no instruction, and may let him just click through the tests as necessary. that way, you can teach him yourself out of the Brown/Dolciani books (or whatever you choose). I wouldn't recommend the Mizzou OHS otherwise, though.
