S.K.
Hi S.,
I am having a very similar problem with my son who is the same age and transitioning into Kindergarten.
He has hit (a girl!), been pushing for no reason, runs from teachers and basically just shuts down and will not respond to anyone making it very difficult for them to deal with him plus the rest of the class. All of these are uncharacteristic behaviors.
Trying to get him to talk is the hard part. One suggestion that the teacher made that has had a marked effect on us was a book recommendation. It's an easy read and the techniques really made sense and my son has responded to certain ones even though I thought he would not. The book is: "How to talk so kids will listen and listen so kids will talk" by Adele Farber and Elaine Mazlish.
This book's techniques are good for kids of all ages, including teenagers, (and it works on adults too!!!)
Maybe this is a book worth taking a look at.
Warmly,
S.