I think that your son is in a very good place and the fact that you are agonizing over this probably means that you will strike a good balance. I am probably in the minority, but leaving things well enough alone was pretty much my motto for K and 1st grade. I don't think that it has harm either DS9 or DD9, who are now 4th graders, and who have moved further and further ahead of their peers as time passes. Of course, my approach may not be ideal for everyone - particularly if there is an issue of pschological harm as reported by some parents here and on other forums. However, in our case, psychological problems were not a concern. Both DS and DD were relatively happy and very well adjusted in school.
Of course, there were instances when the educational mismatch came up in conversation. DS was a bit sad in K when he came home and told me that he won't get to study math until 1st grade. I am not sure who told him that, but I told him that there was no hurry and he has the power to learn anything he wanted on his own and he can ask me questions anytime. That seemed to sastify him and in fact by second grade (when he was accelerated to compacted 3rd/4th grade math), he had somehow acquired most of the math concepts through at least 4th grade level almost by osmosis with occasional questions directed at me.
Both DS and DD were very far ahead in reading by the middle of K, but it really wasn't a big issue. They were free to read whatever they wanted at home. We purchased books and made extensive use of the public library. It helped that there were so much fun and games in K and there was so much room for growth in writing since that was completely open-ended.
Writing ability (including handwriting) was a major reason why I didn't bother to accelerate DS in math until 2nd grade. Even as early as 2nd grade, math involved a lot of writing and proving how you solved word problems and that writing component increases substantially for 3rd and 4th grade math. Furthermore, accelerating him one grade from K to 1st or from 1st to 2nd would not have solved the mismatch for a kid who intuitively multiplied/divided and combined bills/coins well before K.
