J.I.
Talk to him a lot a week or two before the baby comes. How mommy will be gone for a few days, and he will then have a new baby brother or sister. Kids do so much better if they know what to expect. Like someone else said, let him keep to his same routine as much as possible, that is comforting to them.
Something that really worked well for us was to have a very special present ready from the baby to him. When we introduced them, the baby was on the floor in his car seat, and the present was next to him. We videoed the whole thing. He was so excited that baby brought him a gift, and it associates good feelings with the new baby right away. He watched that little video over and over again.
We were so worried about the jealousy thing, but he was so excited, that it never happened. He is a big boy, and gets to do big boy things, like help get diapers and help bathe baby, etc. They are so into being big at that age, that I would emphasize those things.
If he does want to regress a little, starts wetting himself, waking up at night, etc., don't be too hard on him. Some kids, that is how they react. Be kind, and that, too, will pass.
How exciting for all of you! I wish you a safe delivery and a healthy baby.