Belle, it sounds like you're prepared with the DIBELS and EPGY scores. You might also print out the standards for third grade and check off the ones your son has already met. Our school district has them online. And any other achievement scores -- SCAT? My sons' school is more interested in how high-achieving they are than their IQ scores.
They will try to steamroll you again, but you're not going to let a little steam get to you! Good luck!