Hi J.,
Your baby is too young to be prepared for moving, he won't understand what that is. He'll likely be interested in exploring his new enviroment, and long as his family is still there, his familiar bed is there and you keep his routine the same, a 1 1/2 year old is not likely to be traumatized by a move in the least. Toddlers don't analyze things the way that we do, they have no sense of time and can't understand that they will "never" go back "home" - and really may not care.
Congratulations on the new baby coming. It's also hard to prepare a child just turning two for a baby, because they can't understand how this will change their lives. Exposing him to babies won't prepare him for the nonstop demands a newborn will place on his parents. When the time gets closer, be sure that he sees the baby's room and things, which will reinforce the idea that someone new is coming to the family, but don't start "preparing" him too soon. A one year old has no sense of time, and if they know six months ahead of time, they won't believe you anymore - you've been saying for months that a baby is coming and it never shows up! The best that you can do is to set up help for after the baby comes so that your son's schedule can stay pretty much the same. If he's going to start preschool or switch to a bed, do this well in advance of the baby coming. Pick up some fun new toys/books and make a basket of them - these can only be played with when you're busy feeding/changing the baby. Make sure that when the baby comes, you have someone to hold the baby for 1/2 hour so you can devote time just to your son. And don't expect him to become a "big boy" just because there is a new baby. He's no more capable of using the toilet, not crying or anything else than any other just barely 2 year old without a sibling. A baby sibling doesn't make a child more mature and doesn't make them a big boy, he's still just a toddler - don't let other people put pressure on him either.
Good luck with your new home, and have a healthy, wonderful pregnancy!