Originally Posted by galun
we are starting to see some signs of learned underachievement. He is careless with his work because it's so easy. The teacher brushed it off and said yeah I see this a lot in GATE kids, he knows the material. But how about establishing good study habits? Additionally, he told me that he intentionally pretend that he doesn't know the material to blend in.
You are right. Kids need both an appropriate challenge and academic/intellectual peers in order to develop necessary life skills and mental health. Print-outs of the linked articles, with pertinent points highlighted, may be helpful to you in raising the awareness of your local system regarding the needs of gifted learners.

Originally Posted by galun
We are considering advocating to get him to take classes at the middle school. From my knowledge, this has not been done before in the school district.
Are you considering asking for a single subject acceleration (SSA) for math?

Originally Posted by galun
I am in an area that highly values equity, so our request will likely be viewed as asking for special treatment.
You will want to reframe this. Equity is meeting each child's needs. For example, children wear different shoe sizes, and therefore need shoes which fit them personally... even if that may be larger than the average size worn by children of their same chronological age.

This post has links to archived Tamara Fisher articles for EdWeek, including 'Same' and 'Equal' Are Not Congruent Terms.

To avoid the unwelcome stigma of being viewed as seeking "special treatment", you may also wish to avoid use of all forms of the word "entitled". For example, "identified as gifted... which entitles us to an IEP." You might also wish to avoid using the plural terms "us" and "we" , and focus on your son as an individual, "he". He is identified as gifted in school which entitles us to an IEP might become He is identified as gifted in school therefore he has an IEP.

Keep the conversation focused on your child's needs.

You may wish to read through the advocacy tips and resources, especially the section on meetings.

Originally Posted by galun
Any other respected research that will support my case (would love recommendations here if any).
There is a list of resources in support of acceleration at the bottom of this post.