I'm kind of agreeing with Floridama. I know you've read a lot of horror stories about K, but remember, those children whose K experiences have gone swimmingly, tend not to post.....or post as often as the parents who have horror stories to share.
Myself included, sometimes the horror stories are posted in an attempt to seek help/ideas, ect. My eldest DD did okay in K, back then, I didn't even know there were forums like this and didn't really have a need for one!
However, when DD6 went to K, we really had some problems.
I do think in terms of skipping, the school will zero in on the K skill/s that are not mastered. Perhaps you can work with him this summer to get some of those shored up.
I have had some success by asking the school: "What is x child expected to know/be able to do by the end of K, and is x child already able to do these things?
You could end up with a great K teacher, that makes all the difference. Good luck with whatever you decide.