Is your son a rising 7th grader? Will this be his first year in junior high? WIll this be his first year in a new school? Have they already done class placement for next year? Sorry for the questions, I"m just trying to get the full picture.
My first thought is, absolutely, I'd register him for the SAT or ACT for the fall. It sounds like he is not only super-smart but also a good standardized test taker. If your son can pull off super high scores for SAT or ACT, you might also look at SET. Have you thought about DYS? You might be able to bypass the IQ testing if you can pull together a portfolio...
I have struggled with the "what difference would having the IQ number make" question. I guess I would say that If you know what you want (e.g., grade skip) supporting scores are huge. I had no idea how much would be decided by my dd's scores. It was if we were finally "talking the same language" as the educators.
You and I have both "arrived at this party" a little late. Most posters here have IQ tested and grade skipped during elementary... I think it gets a lot more murky once they get to jr. high.