My sister had her second (a girl) when her DS was 18months old. She did tell him he had a brother or sister on the way but he was to young to understand. He was jealous of the new baby at first (some attempted head-stompings and eye-gougings) but they disappeared within a few weeks and then he just doted on her!

I'm sure your DS will be the same, once he gets used to having a sibling in the picture.