Isn't it lovely how the schools essentially control the IEP system, yet pretend it's a partnership with parents? Your DS more than qualifies based on three objective measurements (IQ, achievement, reading assessment) and fails based on the two subjective measures that are entirely within the school's control (checklist, portfolio). This basically gives them veto power. It's only a partnership when you're willing to meekly go along with them.
We ran into the same issue when we tried to accelerate our DD a year. Our DD was only offered a 2hr/day gifted pullout, which was actually a bigger mess than leaving her in her regular classroom, and her RIAS score wasn't quite as impressive as your DS's. We followed the institutional bias against acceleration all the way up to the district director of curriculum, before we gave up and homeschooled.
Based on our experience, don't be surprised if every facet of the institution rallies around itself.
In your place, I might try the foot-in-the-door approach:
"We have here three objective measurements which say he's an ideal candidate for the all-day gifted program. We have two subjective measurements which say he's not. These are, in essence, opinions... is it not possible that they are wrong? Could we perhaps agree on a trial in the all-day gifted class, of say, one grading period? This could provide the objective data we need to answer that question."
