We found achievement testing more useful in this respect than IQ. YMMV, and we weren't dealing with 2e.
Achievement testing also worked for us, with the school using its own testing. Does your school do NWEA MAP testing by any chance? That is a very good test to show what your child already knows. We were not dealing with 2e either, but we did have outside IQ testing in hand before we approached the school. It led to some discussions, but no acceleration until after the school did the MAP testing and saw how far ahead our kid was.
Since your son is already in the GT program, what are your goals? Full grade or subject acceleration? The MAP is great to find other kids who have similar scores who might possibly be grouped.
