I think it's interesting that your DS is occasionally crying when he's in the K classroom and not when he's in the 1st grade classroom. Maybe he's in some way feeling more comfortable and competent in the 1st grade class?
Also, since the crying has just recently been happening, is it possible he's starting to come down with something? Or maybe that someone said something (perhaps innocently, and perhaps minor) that your DS took to heart that made him really feel particularly sensitive about making small mistakes that otherwise wouldn't have flustered him?
