Oh, Cathy, I'm right where you are, with a 5yo ds in K whose teacher just doesn't seem to "get it." As my dh (who, incidentally, is not ds's father) put it, "I don't know how anyone could spend a day in a classroom with him and not see that he needs more." I also feel like there are blinders on!
Truth is, it's easier for teachers to look the other way and decide that the child is fine.
Does your school use the Iowa Acceleration Scale? It's a questionaire for use by schools to determine if a child is a good candidate for a skip. It uses a lot of indebatable facts to assess the child and is pretty straight-forward. I bought the manual (which you can get for $30 or something on Amazon) to check it out ... the only thing is that it relies heavily on test scores (24 or something of the 80 possible points come directly from test scores). Has your ds been tested? Is that an option? Anyway, that's one route you could try to take.
Personally, I wouldn't want to send him to first grade unless it was guaranteed he could stay there if he's successful with it. The jumping around doesn't seem fair to him or to any of the teachers involved.
On the other hand, if he's really a good candidate for a skip and he won't be able to unless he does the first-grade pull-outs ... that's rough.
One of the points schools often make is the "social development" angle. You could come at it from that point: "It's clear ds is ready for a more challenging academic environment, but we'd like to keep his environment as consistent as possible through this change. So we'd prefer that he either be placed in first grade for the afternoon and stay there for the rest of the year, or that he be skipped directly into second grade in the fall." Something like that, maybe?
Do they do subject acceleration in the classroom, or any enrichment at all? At my ds's school, they don't do a lot of full-grade pullouts, but they do break down reading groups fairly well -- ds works with a group of 5 Kers brought in from the seven K classes. We've also pushed the issue and they've agreed to give him advanced work every day of the week, not just those two days. Do they do any differentiation in the room at all?
Your ds sounds very bright to me; it's too bad more teachers aren't able to enjoy these kids and see them as a difficulty rather than a challenge. Welcome to the wonderful world of advocacy! :-)
Mia