It sounds like you need more information about your son, and if his teachers are saying that he's "pretty exceptional even among HGT kids, then I would probably take that data point and run with it. An IQ test can give you a relative measure, true, but it can also give you great information about your child's strengths and weaknesses. I'm always amazed at what people can pick out of the scatter of numbers across sub-tests.
That said, aculady's advice is good: you wouldn't even need an IQ test to apply for DYS. If you have achievement (with Explore) you could apply with a portfolio and see what happens.
You ask "is there a reason to do it?" Put it this way, if your child turns out to be PG, wouldn't it be great to have a consultant to help and a group of parents who've been through some of your same experiences?
Have you ever not shared something your child did with another parent because it seemed TOO advanced?
