So if your son's teacher is wishy-washy on the subject of what your child knows, you might ask in a sweet voice, "Would it be possible for you to give DS the end of the year tests for 2nd grade in order to determine what he knows?" This is something that she can do on the spot, so to speak, without a lot of additional work.
Well, hopefully, she can but this hinges on the district policies. Ex: our district doesn't really want to accelerate. So when DD's second grade teacher supported a grade skip, she was pretty much shut down.
To this day I'm to understand that there really aren't any tests to assess whether or not a student is promoted. Huh?
Now, that's actually something I can turn around and bury my principal with; but I won't. Any gain that could come of it is not worth the fallout. The point is that you are heading up a slippery slope.
I agree with JB and E-beth and will just add another simple but important thought: When in doubt, keep you lip zipped.

If you are getting to a place that is non-productive, you will know it. In fact, if you are starting to become agitated during the conversations that's a magic sign to stop talking and start taking mental notes. Hash it out later and you never have to make a decision or accept a solution the first time you hear it in a meeting.