My older daughter was exactly a year old when her little sister was born. I taught her that there was a baby inside and told her that she was going to be the big sister. She learned to point at my belly whenever we asked her where the baby is.
I think that at 20 months a child can understand a lot. You can explain that soon his sister is going to be born (though, of course, he can't understand the concept of being born) and that she will come home and be home with you for always. Talk about what she will be able to do (cry, drink milk, suck a pacifier) and about what she can't do (walk, say mama, eat food). Explain that babies are very delicate and that we need to be gentle with them. Tell your son that he will be able to help by bringing a diaper when you need to change the baby and involve him in the baby's milestones (smiling, laughing, rolling over, etc.) You'd be surprised how excited a child that age can get about it!
As the previous answerer mentioned, you do need to be careful, because babies don't really understand how to be gentle & they can really do dangerous things. Make sure to protect your new baby, even by putting her in a playpen if you have to leave the room, even for just a minute.
There's a lot more I can say, but I'm sure that's plenty - just one last thing... When you bring the new baby home from the hospital, know that you are bringing your son the best gift he could ever have asked for - a sister who will be his friend for his whole life.
Rachel Inbar
www.FertilityStories.com
mom of 5 and sister to 4.