D.B.
S., is your son pretty young for his grade? One of my daughter's classmates was born in August, prematurely, and he just wasn't up to the same level of concentration the others were because, for all intents and purposes, he wasn't supposed to be in that grade. It sounds like a bad thing, but it's really not, to ask the teacher if he should be in kinder again.
If that's not the problem, maybe he just needs some adjustment time. It's tough going from summer vacation back to school where you have to concentrate so much. Has he been tired or irritable too?
I hope you find your answer! Good luck!