Use a picture system for him to understand. There are some pictures at do2learn.com, but it might be more "real" to him if you take a digital camera and take a picture of a baby, a diaper, a pacifier (if you plan on using one), bottles (if you plan on using one), a baby blanket. Then print the pictures, put them in a book and talk to him about them - there is going to be a baby, he will sleep (where?, in a crib in what room?), he will suck on a pacifier, he will drink from a bottle, etc, etc.
If he doesn't understand pictures, then objects are the fall back - put all of the items in a box and use the objects to talk about it the different things that will happen. You will need to use these "stories" multiple times so that he gets used to the idea and understands - the more times he hears it, the more he will retain.
My 12 year old has moderate autism - if you have any more questions, let me know.
at 8:41 PM on Mar. 1, 2010