Hi longcut, welcome! I cannot answer all your questions, but I'll give some of them a try.
First, the CogAT. I would not discount a high score like that. My HG+ son scored similarly. It is not an IQ test, but it is a test that shows that your child will likely do well in the GT program the school offers. I see more concerns on this page about the test missing some over-thinking HG kids, but that doesn't always happen either.
It's great that your DS's teacher is so supportive! You ask about acceleration. I think that if your kiddo is working 2 years ahead across the board, it is definitely something to at least discuss with the school. It is easier for teachers to differentiate when the base level of the grade is closer to where the student is working, and there is more of a likelihood of additional company working at that level.
IQ testing -- is it necessary? Well, that depends. Is it required to gain access to anything you want for your kid? If you want him to be in a particular program at school, will the school even consider IQ testing? Does the school provide the testing for you? Would you want that if they did?
I am not sure if the IAS will substitute something like the CogAT for the IQ portion of the survey, but if you are really considering a grade skip, I think an IQ test would be able to give you more information about whether your child would be a good candidate. A certain number of points are added up for each section of the IAS. I would imagine it's possible that you could still end up with a "good candidate for skipping" score even without an IQ test if there were strong scores in the rest of the survey. But I haven't seen the IAS for years, so I cannot say for sure whether it is a required component. I do highly recommend using the IAS when considering a grade skip, though. Helps to convince parents and the school one way or the other with all the objective considerations.
We did use IQ testing successfully with the local school to get some differentiation in kindergarten (when at the time no GT programming started before 3rd grade). The GT coordinator at the school was very helpful and really understood what the scores meant. Many teachers have no clue what IQ results mean, and so they would be useless. I would ask his current supportive teacher or his ELP teacher their thoughts on these issues, if you feel comfortable. or if there is a school or district GT coordinator, maybe that's a good start.
The Explore test is a great out of level test to use to see where your kids are, but I think that may not be an option anymore (or else this might be the last year it is being used). Check with Belin-Blank to see how long this test will be supported. It is a relatively inexpensive way to see how different your kiddo is.
Good luck. Sorry for rambling.
