Congratulations to you and your family, A.! I know this is a very exciting time. You have already received some excellent advice!
My girls are 18 months apart and the third came along 20 months later. I found the more involved you allow them to be the less animosity they feel. I gave my 1st daughter a baby and we did everything together with our new babies. When it was bath time we both bathed are babies, fed them, changed them, rocked them, and put them night night together. This really allowed her to feel more involved. You could see the pleasure on her face when she was being a "big girl" and helping mommy with the baby. By all means, when appropriate let your son help with the new baby. If he doesn't seem interested, let him do something special with dad while you tend to the baby. This helps let him know that he is still special too. Of course, this works both ways, if you have something special you do together at bedtime, bath time, etc. make sure you continue and have someone else take the baby so you keep that "special time" with your son.
Also, I highly recommend that after the new bundle of joy arrives you request all visitors to acknowledge your son first (eventhough, they are anxious to see the baby) and if they bring gifts have them bring a little something for your son too. This helped tremendously when my 2nd daughter arrived and worked just as well when number 3 came along. As a back up, I always kept a few small things stocked in the closet, set up in gift bags to give to the other kids just in case someone forgot or it just wasn't in their budget! I wish you and your family all best! God Bless!
J. F.
The MOM Team
http://www.4MeAndMom.com