J.K.
The novelty has probably worn off and/or he was tired or getting cold. So many things can cause this. I wouldn't pull him out if he's enjoying it and just having a hard time transitioning in the morning. Unless he's having a hard time socially or with the teachers and it isn't a good fit, he'll probably go back to liking it again. Really normal.... I wouldn't worry.
There are plenty of days that I don't want t go to work! :)