R.D.
Deep sigh. At your situation and some of these responses.
Been there. My son is 13 now, 8th grade, public middle school. I'll spare you my "what's wrong with public middle school speech" and the "one size fits all educational model does not benefit every child" speech.
Yes, move him. Move him right now.
I know that Montessori is a system that is taught at an early age, but trust me, he'll catch on. He NEEDs a change of scenery. The current one hasn't worked for (let me guess) many years. And it's only gotten progressively worse. He's NINE. Get off the train now. Take a break. Give him Montessori. Montessori is phenomenal and if we had one that we could afford, my kids would be there.
You are very blessed to have a husband who is open minded enough to try something different.
I'll come back and post some resources later.
Edited to add: The Montessori classroom works in a very different manner. Give it a few weeks. If you and the teachers agree it's not working, then consider another option. But try first.
6th grade: Year round, very expensive tutor (which he hated, you should look up that post of mine) and he went from a D to an A in math. Other classes, so-so. The 36 kids per 1 teacher environment is NOT ideal for him.
7th grade: hired a college student (nice young man) to do homework alongside him to keep him on task. DS hated it (liked the guy) grades plummet because DS was absent (illnesses) and fell behind on late assignments. D in 2 classes. He lost his cell phone for many months. THAT really motivated him to do better in school. I've seen improvement.
8th grade: Finally, more self-motivated to sit and do homework without prompting (arguments/drama/meltdowns), but totally coasting academically. Not pushing himself. He's getting the bare minimum of a lame public school experience. He begged not to have a tutor. We are giving that a whirl this year. He's keeping up.