All good comments. I'll sing the same tune. We skipped K with one and early with another with winter bdays (but with girls).
I'll say (1) the behavioral piece will matter most probably. As you get closer do you think your ds can focus, task completion, resist instant gratification behavior etc. (2) I think trying it may be easier to undo than not. You could always skip K and stay in pre-k style if you think he would be unhappy with the structure or go to a montessori K with a little more freedom. (3) The big question for us with the dd who is more boy at heart... in a year will she be ready for the structure and no play of 1st. Will she have the ability to follow multi-step instructions. She's hg+ but... she's a dreamer. But look, I've been through this before and I bet everyone will say the same... thinking about all the possibilities can drive you crazy. All you can do is take the scores and your knowledge of your child and take your best educated guess. Know that it isn't locked in stone.
Remind me of this when I freak out again :-)
