I agree you may want to hold your S back from starting K. Boys especially may need a little extra time to mature. Especially with our life style and different states having different cut offs. Our youngest turned 5 on the cut off date where he started K. Which meant he started K at age 4 (turned 5 a couple of months into K). He did fine up until 4th grade. He was really struggling with his grades. We moved half way during the school year and put him back to 3rd grade to finish. One reason was because we could see he was struggling, but the biggest reason was because all the the kids in the 4th grade at the new school were already 10 in Jaunary and S would not be turning 10 until the following fall.
We are SO glad we did this. He now makes high honor roll every term and has been chosen to represent his grade in Math field day for the past two years.
I am going to recommend some things that I have read and used.