Welcome Laura!
I love that your son advocated for himself! Good for him. Too bad the school isn't helping him though.
I'm a little confused by a couple of your comments. You say that your son was accepted in a GT public school. Is the curriculum advanced in any way? From what you said about the pullout, it doesn't sound like this is a full-time GT program. Are there other options in your area if this school doesn't work out so well? How is your son in other subjects (in case a full grade skip might be easier)?
Does the district have any policies on acceleration? Maybe you can find something on their website.
Was the homeschooling afternoon dedicated to math? Where does your son fall on the scale of MAP results for higher grades? (I'll try to post a link to the MAP tables and you can find where his score falls in 3rd/4th/5th.) Here's the chart. Look on p. 145, and you will see end of year math data. 99% will encompass everything 217 and above for grade 2. So, if your DS scored 217, he was in the 89th percentile for grade 3; if he scored 222, he'd be in the 95th percentile for 3rd and the 78th for grade 4.
MAP norms charts Did the school give any indication as to why they wouldn't let your son go to higher math? When we tried to get higher math for our DS7 at the beginning of the year, we got the excuse of "the classes don't meet at the same time" and in our case we also got, "his writing is so bad he wouldn't be able to keep up."
