I did it! I finally got up the courage to ask about DS trying out 2nd grade math! Even better, the teacher didn't outright refuse.

She said he's definitely ready for 3rd grade math, so she isn't worried about content, but she'd have to talk to the 2nd grade teacher (whose wife had a baby yesterday, so it won't be until after spring break) and the principal, who in the past has been very supportive of my kids. I told her that I didn't want to make it a big deal, and she said that it wasn't a big deal at all. I reiterated that I knew he'd been getting frustrated with the problem solving worksheets and that I thought a little frustration was good, but I thought that letting him go to 2nd grade math might be good for him since he would be getting direct instruction. So, obviously, nothing has been agreed upon, but at least I got the courage to put the idea out there. So, yay for now!