R.K.
I am a Kindergarten teacher and this is a typical behavior for children of this age. I would just be aware of the situation at his age and try to allow him to participate in activities as he gets older. Socializing in the classroom is a key thing that teachers observe at this age so, he must be very shy at school for his teacher to bring this up to you. My concern would be that if he continues to remain "introverted" talk to him and find out why...there is some reason that he is not comfortable being himself there.