Presuming this is the first time your son has had math issues, I would again see it as a style conflict with the teacher.
Alternatively, presuming that it ISN'T the first time (and I suspect that it's just a worsening of an earlier phenomena, given that the school was relatively unwilling to do an accelerated placement, yes?), I'd explore whether or not this is a matter of:
a) uncovering a missing set of study/learning skills which have only been revealed now that your DS is finally seeing novel and challenging material, or
b) a previously masked learning disability of some sort.
The first thing that I would do as a parent-- and I've been (kind of) in your shoes when my DD took geometry-- is figure out what kind of instruction is taking place in class.
Offer some direct instruction AT HOME-- using the class textbook. Does your DS have homework problems? Are they difficult enough that he needs to write out his work? Is this material that he's seen before? What KINDS of errors is he making? Are they computational? Or conceptual?
The only way that you'll learn the answers is by observing his work and what he's doing.
I had to do this with my DD in geometry-- I literally "taught" her the material (rather than relying upon the school's notion of "student discovery" as a primary learning methodology, which I just... well, words fail). I used a white board, sat her in a chair, coached her on HOW to take notes on what I was showing her, the whole nine yards.
That was the only way that I could learn
what the nature of her problems were. She was more than capable... she just wasn't getting the instruction in the manner that she needed (didactic, Socratic) in order to truly make sense of it at all.
