One of the most difficult discussions to have is the one where you broach the topic of the autism spectrum. Something that I regularly hear from the parents of the children who participate in my autism social skills groups is concern about how to have "the talk" with their child.
While there is no "one size fits all" way to successfully broach the topic, there are some values that you can establish in your home that lay a foundation for graceful and complete self acceptance and an opening into a deeper and positively biased exploration of the neurological differences that make your child uniquely who they are.
One thing you can do, early and often is to lay a fountain of pluralism and acceptance as a deep family value. There is no way to soften the importance of this. It will be difficult or impossible to be authentic when trying to convince your child that you honor the differences in their brain functioning that make them unique if they have heard negative things said about other people's ethnicity, sexual orientation, or religion. Autism is a spectrum, but pluralism as a value is not.
The other side of pluralism is universalism. As we recognize and honor the differences in everyone, the universality, the alikeness of the human experience becomes evident as well. While people may look, eat, dress, or talk differently, they love, hurt and laugh the same.
This is not easy, but the thing I keep coming back to in my mind is that, most likely your child senses that they are somehow different than many of the other children around them. In that context, it must in some ways be a tremendous relief for them to be told that their difference has a name, that there are other children like them, and that there are skills they can learn to fit in better in the ways they choose to.