Does he like to read things? Or watch TV? My mom knew I'd read anything I would find, and so she put something on Aspergers on the table where she knew I'd find it, and sat at the table while I read it. Of course, that's just my experience.
To prepare though, remember that AS is an ability more than a disability, and don't give him the "special" line. Don't act like this makes him any less able, or any less anything, though I doubt you would.
A good "do" might be to make any information on AS you have available to him. He might get hungry for information on AS, and it's good to feed a thirst for knowledge.
It's good that you're telling him, I think. I'm glad my mom did, that's for sure.
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FOUR
Four is the only number which is itself has the same number of letters as it itself is.