Hi T., I am both a grandmother and a teacher.
First, my children found a book that phonetically teaches children to read. They each worked with their children, and had them reading--real reading-before they started Kindergarten. Each of their kids could have skipped a grade, but they chose not to. Now each child is a strong student with good self-esteem, and some have been passed up, in reading skills, by the other students.
What is harder to change than reading skills, is our self concept. Had each been put ahead, they would have been with children one year older. That means that they are always behind in growth, social skills (that are developed as the brain develops, not as how parents wish it to be!)and even learning abilities.
You might drop by the class room and see what size todays kindergartener's are, compared to your son. Watch them, and notice the things that they have learned, half way through the year. They will go into 1st grade with that whole years training, that your son will be missing if he skipped ahead.
One of my daughters had to keep reminding the teacher that while her son read well, he needed all the other skills that Kindergarteners learn, that only she (the K. teacher) could teach him.
As a teacher, I can tell you that the older, smarter children in the class are generally the leaders in scholastics and in games, etc. Put a group of little kids together, and they usually sense who is older, and follow them. Kindergarten teachers can usually tell, within a few weeks, which children are the older, because their brains are able to process sociability on a higher level than the younger students.
As a Mom, I sent my oldest to school a year early--(where I was living that year, the cutoff day was Dec., but where I would live the rest of my life, the cut off day was Sept. I chose to send this Oct.29th baby to school that year.)
Yes, she is smart, but she always had to try her best, which makes her an intense, high-achiever person today. Also, in sports, and emotional development, life would have been that much better for her had she had another year at home.
These are my experience. Hope thay give you some insights.