Hi mom2twoboys, welcome!
In my experience, K is a very important year. This year, kids adjust to the new 5 full day school, learn how to sit well, learn to socialize with other kids etc. There is more to learn than just academic.
My ds7 was reading at 2/3 grade level and doing double digit math with carrying and borrowing when he entered KG. However, he did not like to write due to asynchronous development, would not draw or color and he would interrupt when the teacher was talking. It took him a little more than a year to catch up.
He was the highest level in class. However, there were kids that read at 1st/2nd grade level that the teacher paired him up with. In math, the teacher challenged him by plugging in bigger numbers in the same word problem for him. He learned to add and subtract double digits in his mind and explained his answer using words. Academically, he was somewhat challenged, but he caught up quite a bit in his fine motor skills and learned to behave in a group setting. He had a good teacher. I really appreciate her effort in keeping him challenged.
I have always known ds is advanced. I did not test him until he just turned 7. I have known a lot of people here tested their kids very early. IQ test score are not very stable until they are 6/7 years old. We paid a private psychologist to test him.
I personally would wait until your ds gets into KG and see how he is doing, then talk to his teacher. If he has a good teacher, she would have figured out he is advanced and would differentiate in class. She might even bring up the issue of testing him before 3rd grade.
I was nervous like you when ds entered KG. I totally understand how you are feeling right now.
