I would suggest that you check your state special ed law and regulations. If your state provides that each student has a right to reach their potential, you may have an argument given the discrepancy between achievement and IQ. But, if your state is like most, and they only follow the feds and guarantee access to the curriculum and appropriate progress (usually defined as grade level by the courts), then it will be hard to get an IEP and even a 504. Since you have teachers on board now that recognize he benefits from extra time, I would call a meeting to see if he can qualify for a 504. Their comments will be crucial, You will want to demonstrate that he has been doing as well as he has due to their informal allowance for accommodations. You will want to get these in writing so that other teachers that are less enlightened will also give him needed accommodations.

Here's more info on 504's:
http://www.greatschools.org/special-education/assistive-technology/1526-Section-504.gs?page=1
http://www.ncld.org/on-capitol-hill...on-504-of-the-rehabilitation-act-of-1973