Natalie started taking notice of people of different races (and calling her new best friend in preschool "the little brown girl," when she couldn't remember her name at the beginning of the year), I think I said something to the effect of "isn't it cool how God made people all different colors of the rainbow? Maybe you can ask your friend her name again tomorrow."
And then hoped and prayed I said the right thing.
She has stared at people in wheelchairs before and I said something about how some people use wheelchairs to get around, and some people walk.
I don't think you need to get into details regarding race and specific disabilities at this point, because if he's really young, he won't understand anyways. At that age, I don't think he's passing judgement so much as just being curious about something different than what he's used to seeing. So he'll follow your lead on how to react, you know?