In many places, K is optional and the district is not required to offer it (though almost all do offer K). If that is your case, your son needs to finish a "real" grade at an accredited school.
I would consider his maturity and what he would like (if one route means staying with friends, how it affects sports, etc.). I know folks will rip me for the sports comment, but when my very small 2nd grade kid was a grade ahead and in with kids - some almost two years older - in rec basketball, it wasn't pretty. She repeated 2nd when she switched schools (private to public, different birthday cutoff).
I think that most kids around here could skip K without missing any "academics". In this area, kids stay in their "correct" grade more for social reasons than for academic ones. Consider the types of kids in your district and if he would have some intellectual equals (or near equals) in his grade.
