Is gifted public an option in your area???

I am feeling less and less comfortable with your current choice (sorry)... of course, I am looking at based on my personal experiences (!!! two boys) and yours might be different :-)

You make a good point this needs to be the right decision now... if he is not way ahead in reading, maths and writing prior to the start of K you might be OK for a bit, with enrichment at home as needed.

Best of luck!