Aggressive Preschooler

Question:

Apparently for the past “few weeks” my almost 3-year-old has been pushing and shoving his classmates in his preschool. Today the teacher told me that he poked a little girl in the face and made her cry then refused to look at her when the teacher asked him to tell her he was sorry. I don’t know why she didn’t tell me this when it first started happening, so I’m not sure exactly when it started. The teacher says there is generally no interaction between my son and the victim prior to the pushing/shoving/hitting. He’s just spontaneously doing it.

I have never seen any behavior like this out of him and I’m with him all day long (except for the two mornings he goes to school). He has been a passive child since birth, no signs of aggressive tendencies at all. Before he started school this year and even up until Thanksgiving, he wouldn’t even hold his own place in line at the playground. He just let kids push him out of the way and go in front of him. I wanted him to become more assertive, but I sure don’t want him to become a bully.

Sam is an only child and we are not going to have any other children. What can I do to nip this behavior in the bud and is this a normal phase even for a well-behaved child?