Hi Green. Welcome!
There has been a lot of discussion of this topic in the past, so you will want to look through the archives. It should be helpful.
I have a DS8 who skipped K, but he is very tall for his age so I'm not sure that I come at the issue the same way others might. While there are a lot of factors that come into play when making a decision about whether to accelerate, to me size should be a minor factor. There are a lot of people who will disagree with me, though. Pros for skipping regardless of height are that your child will have a chance to be challenged and to learn, and could very well be socially successful regardless of height. Also, depending on how small your child is, he could wind up being one of the shortest in his class regardless of whether he is accelerated or not. Cons would be that your boy might stand out as different physically and it may make it more difficult to be at the top of the heap in athletics. Whether any of this matters to your family, though, is dependent on your view of what schooling and childhood should ideally be. For instance, sports are very important to some families and optional for others; academic challenge is important to some while others find social relationships more important. So, it can be hard to give blanket advice.
FWIW, I think there is a lot of variation in physical development at that age (e.g., probably a lot of kids don't ride a bike without training wheels yet), so you may want to discuss your concerns with his teacher and you may want to go in and observe his class.
What about letting him do private K this year (he's already in the class, right?) and then seeing whether you want him to repeat K in the public school next year or whether you want him to go to public 1st. Then you'd have a whole other year to gather data and observe.
