Your battle is partially won if his teacher is supportive. I would grill her in person (might be reticent in writing) for every bit of info about the gifted program in 3rd grade, including qualification criteria, testing, etc. If entrance is not based on testing or even if it is, it may not hurt to have some documentation of your own. Johns Hopkins University's Center for Talented Youth (JHU-CTY) provides SCAT testing for 2nd graders year round for a modest fee (under $100).