Junior, I am happy to share our experience but I'm not sure how helpful it will be. My DS is not as introverted as your son sounds (from your description) although he is reserved around children his age. He is gregarious with adults. He is not disruptive, in fact, he is extrememly unassertive.
When DS had just turned four his preK teacher recommended that he join the Kindergarten class at his private Preschool/Kindergarten. I readily agreed hoping that DS would find playmates among the older children. Unfortunately, the Kindergarten teacher was not happy about the idea (I was not aware of this until much later) and set about systematically excluding my son from activities in her class. She even told him he was "not really in Kindergarten" because he wasn't 5 yet. DS did not receive the support and encouragement he needed to join the social circle of the Kindergarten. The teacher did not want to bother with him in any way. If he had trouble with any task she just told him to skip it because it was "too hard" for him. The damage she did in just a few months has taken at least as long to undo.
My DS is now attending Kindergarten for a second time in public school because of a state law preventing him from early entrance to first grade due to a November birthday. He is much happier this year and feels like a true member of the class although the academics are far below his level.
So my advice to you is to make sure that the teacher is absolutely willing to work with your son and include him as a full member of the class before you commit to the early entrance plan.
My guess is that your son will have plenty of opportunities to learn to get along with other children and tolerate uninteresting activities in either setting.
Cathy
