My nephew is becoming a Bully.. help!

My Nephew is 3 years old, his mom is pregnant and he is becoming a bully kid... hitting other children just because, wants to be picked first in everything, if not, he will yell to his teachers; he doesn't have respect for them either. Most of his teachers say it's because of the pregnancy, but at home he is the most sweet and well behaved child. His mom talks to him, and punishes him and makes sure he understand why she is doing it, and it works for a couple of days, but then it all starts all over again. We are afraid he will become a bully and we do not know what to do... please, any suggestion?