Although I am not pregnant, I just wanted to say congratulations to you! I am sure that everything will work out just fine. I would think that as you get a little bigger, and your son gets a little older, he'll be able to better understand.
I know that when I was pregnant, we had 2 dogs (I understand that they aren't as complex as humans:)) and we got a baby doll that cried and made noises. My husband and I would walk around holding the doll so that they wouldn't get jealous. Maybe try that to get your son used to hearing the sounds that babies make.
At any rate, good luck to you!!!!!