DS2 was born when DS1 (dx ASD) was 5.5 years old. I told DS1 about the baby when I was 2 months pregnant, because I couldn't do as many physical activities with him anymore. We had a beat-up anatomically correct life-size newborn doll (the type used for childbirth ed) whom DS1 named "Thomas." Thomas became an important member of our family during my pregnancy -- he went everywhere with us. After DS2 was born, Thomas got the outgrown newborn clothing, and everything I did for DS2, DS1 would do for his doll -- bathing, feeding, changing, esp. changing, LOL.
The Dr. Sears book "Baby on the Way" was helpful, too. DS1 kept going back to the picture that showed a mother's profile w/baby in utero. I also pointed out babies at the grocery store, etc., so that he would have an image to work with.