The Delayed Reactor
What he's feeling: I'm cool with school. I'm cool with school. Then, a few days or a few weeks later.... Wait -- where's my mommy?!
Your response: Talk to him about the things that he enjoyed so much at the beginning. Speak with his teacher so she can give him a little extra attention. Also, try to develop a quick goodbye ritual, like a handshake or a wink, that you can do as you leave.
P.S. It's not that your child suddenly decided he doesn't like school. He was just so caught up in all the new experiences that he didn't even realize he'd been away from Mom and Dad. He'll bounce back.