How about starting with a story? My Brother Charlie, by Holly Robinson Peete and her daughter Ryan (Charlie's twin), is a wonderful story. It is well-written, nicely illustrated, and does a nice job of helping kids understand more about autism without it being too scary. Also, on the very last page, Ryan writes about why she helped write the book and gives great ideas for kids at school to be more accepting of kids with autism.
I have twin 2nd graders and I will ask them what they would tell their classmates about their brother with autism. They won't be home until tomorrow night, but will check back in when I have had a chance to talk with them.
What a great opportunity for you, your son, and those in his classroom!
Proud mom to 3 beautiful girls AND one very lucky little boy :)