I haven't had this exact thing happen to me, as my 5 yo has ALWAYS been pretty clingy, but she did go to gymnastics at age 3 without a protest--and now, she wants me to stay in the room when she takes a 2 hour jewelry class at JoAnn's...If you have the inclination, the book "Touchpoints 3-6" by T. Berry Brazelton is awesome, as it describes the stages they go through. There was another book I read about age 5, and it also described how they "cycle" back and forth between independent/dependent, etc. so it's probably very normal. But I would talk to him about it and make sure nothing in particular happened to spark this off--did something or someone scare him, for example. It also sounds like he just wants you to listen to him and believe him.
At any rate, it may sound crazy but I say enjoy the extra time you'll get with him, because he'll be a teenager before you know it and pushing you away!