Shy

Caden has become a frightened, cling to my leg and hide his face kind of shy boy. I don't understand how this happened. At home, at school and in familiar places he is his normal rambuncious self. Loud, running around, acting crazy and talking non.stop...did I mention talking non stop?! For instance, he is in love with police cars and fire trucks. I took him to the fire station that I am going to be volunteering at to show him the trucks and the ambulances and he just FROZE. He would not let me put him down and he just buried his face in my shirt. A fire fighter even came up and tried to talk to him and he wanted no part in that. I get that he may have been a little overwhelmed but to not even look at the fireman (who he idolizes) or go into the fire truck (which he talks non stop about)? Another instance, we want to get Caden involved in something since Isabel is doing dance. He chose martial arts so we took him to meet the instructor and see if he likes the class before we paid the fee. Again, he wouldn't let Brian put him down. He wanted no part of it. I get that it is normal for kids to be nervous in new situations but is it normal for them to act like this at his age? Does anyone have any advice about how I should be handling this?