B.C.
He's jealous of his baby sister and the attention she gets. Have him be your helper as much as possible. Go out of your way to point out all the great things he can do that the baby can't do yet and he can help teach her. He's your big boy and a great big brother. He also might be a bit intimidated about school coming up. Make it sound like he gets to go on a wonderful adventure that the baby's too little for yet. My son went through a stage when he wanted to copy everything I said, so I made sure I'd say something that was good to hear "Mommy's always right!", and after a few minutes of that I'd tickle him and the game was over.