Preparing my 3yr old

My 3 year old is an only child and his dad and I aren't together. He's an only child between me and my husband & he is his dad's only child. He's the baby of the family and is the center of both our families attention. I'm only 5 weeks 4 days but I'm nervous to tell him. He's very smart and I think if we present the baby as a positive he'll be excited. Then when baby gets here it will be a different story. He shows jealousy of my husband when we are affectionate with each other. Does anyone have any recommendations to prepare him?