Many thanks for all the suggestions! I do think there are a few things going on here, and the suggested resources/techniques are worth trying. I talked with him about the underlining/crossing out technique and he nodded like that made sense to him.
It could be the transition to constant story problems has thrown him, especially because, as someone pointed out, he's used to easily being quite good at math (although this is certainly not the first time he's had to wrestle with it, it is the most notable). I'm seeing some evidence of sloppiness (ex. it's pretty clear that he wrote something down wrong or didn't take enough time to set up the problem). So we can work on that...
Wondering about the teacher's role in all of this, though. Certainly, he's got some things to work on and we will support that happening, but wouldn't you think that she would have reached out to us or suggested something for him? Because he's been accelerated, she knows about his math grades for prior years and that he has always been at the top of the class. I'm thinking as a teacher, I would have said something when a student like that was pulling Cs. Or, perhaps all the students are getting Cs and he doesn't stand out? (As DH said, it's hard to believe that he's the only one struggling this term, given how he's done in respect to classmates previously.) He also just did a ton of extra credit pages (all on his own, this is something he wanted to do to try to raise his grade) and she just said "you should have been doing these all along." Yes, but not exactly encouraging. Unfortunately, he also missed several problems on those (not all, some pages were 100%s). And she doesn't say anything like "it looks like you are having trouble with x, go look at these pages again."
