Hi, I am a Mom of DS5, he is currently in K. He excels in math (doing 4th grade math now). Your son is very good in math and my guess would be that he would for sure tested in 99%, probably higher.
Whatever you are doing is for sure working
It's great that he is happy in school. Is his K half day or full day? How much is his teacher aware of his math abilities? K work is way too easy for him and he could use much challenging work. I would suggest Zaccaro book for elementary students.
It takes time to get used to the gifted label. I suggest reading Ruff's book. It explains quite well, how gifted children differ from other kids as well as each other. They really have different needs and smarter they are more difficult things get.
