My girl is 9 and has sensory processing disorder, not autism. However, my 6 yo son has PDD-nos and the two disorders share many common characteristics, including that nearly identical areas of the brain are affected, though to different extents. She was diagnosed a year ago, and this is how we explained it to her (and also how we explained her brother's too):

We showed her a very simple diagram of the brain that our neuro gave us. The dr had circled the affected areas of the brain (social cues, emotional regulation, sensory processing, mood, motor planning). We talked about how brains usually work for most people in simple terms(that input travels through the nerves up the spinal cord to the brain, where messages go to the proper parts of the brain and your body is told what to do). Then we broke it down in terms of things that are hard for her, and explained how her brain doesn't always send the message to the right spot, or doesn't understand the message, or sometimes the message gets blocked and her brain doesn't even realize a message came in! (And for her brother, we added that sometimes the brain thinks things are really important, and can't stop sending a message about them--which is why he fixates on something.)

It worked for her, because she loves all things science-related and she is a very visual learner. We'll use the same approach my son later--right now he doesn't realize he's any different than anyone else :)

Another thing I found helpful, and my daughter found wonderfully helpful to understand her brother's behaviors, was this mom's blog and the way she explained her son's autism to his 4th grade peers. http://momnos.blogspot.com/2010/03/on-being-hai...

My six year old daughter was diagnosed early this year with ADHD and PDD-NOS. She may have been a little young but I figured I would start the conversation. I felt she was smart enough to know that she was different from other kids and wanted to try to explain why. I told her that she was born with something called ADHD and PDD and that it was a gift that God had given her. My daughter is very tactile and can manipulate anything (a pipe cleaner, wax stick, or even a piece of tissue) into an animal or bird (her preference). She is also very passionate about talking about birds and knows so many details and facts that most people do not. She is also very artistic. I explained that the reason she can do all these amazing things is because of the ADHD and PDD. But with this also comes struggles. I told her that everyone has their struggles and even discussed some of my own. The ADHD and PDD were the reasons she had a hard time in school and it was more difficult for her to make friends. I told her that no matter what we loved her and are always here for her. I asked if she had any questions and she didn't. That was over the summer. I'm sure I'll have another conversation with her again but there seems to be a deeper understanding of why she behaves the way she does. I wish you luck in your talk with your stepson. Definitely not an easy conversation to have.

Your child already knows he's different. Explaining that there are people, just like him, that are different than most others, and that it's ok to be different, can go a long way in helping him cope with a world that doesn't "fit" him. If you then explain that a "label" is about helping people who are a part of the large group understand and help those who are different, it can help him feel better about getting diagnosed.

If you understand bell curves, you can use the image to explain that for everything... height, hair color, learning speed, social perception... any characteristic or skill, there are people whose skills or characteristics are like most people, and those who are different than most. The autism spectrum is one side (not "higher" or "lower") of a curve that involves seeing physical world details vs. social details. There are people scattered on the far side of social detail perception, too, that are so focused on social interaction they do poorly in work and such because all they can concentrate on is how others' looks make them feel.

And, "normal" people (e.g. the majority), have a balance of good feelings from structured time and environment, physical stims (there's a reason for rocking chairs and pen clicking, etc.), and social stim.

When he gets that, then he can start to develop understanding about why he needs to learn the things that give others' good social stims (e.g. body language), just like he'd like them to understand and support his routines, sensory needs, etc.

THERE IS A BOOK THAT I RECALL THAT SAYS SOMETHING ABOUT A CAT THAT HAS AUTISM I AM SORRY THAT I DO NOT RECALL THE NAME IT IS AT BARNS AND NOBLE. IT EXPLAINS WHAT IT IS AND WHAT TO EXPECT FROM IT IN A CHILDS UNDERSTANDING JUST GO TO A BOOK STORE AND YOU WILL FIND A GREAT SECTION. MY KIDS TEACHER BOUGHT A BOOK TO EXPLAIN TO THE CLASS WHAT IT IS. AND MY SON HAD A BETTER RESPONSE FROM THE KIDS WHEN THEY UNDERSTOOD WHAT WAS GOING ON AND WHY MY CHILD WAS DIFFERENT.

All Cats Have Aspergers
It is a wonderful book that explains things simply and clearly, with adorable pictures and a very accurate portrayal of strengths as well as challenges. I keep a copy in my classroom and use it as an introduction when staff or parents who have no prior background ask for guidance.