I talked with my son when he began noticing and asking questions. The thing is, he already knows he's different. So it's best to talk with him and answer any questions he has. I told my kid that brains are all different. Asked if he'd ever noticed how some people are really good at math, some really good at reading, or art, etc.... And explained that brains are all different. Some brains are good with math, or science, art, music, puzzles, remembering things, while some brains are good at one or some of these, and not so good at the others. Some are very good at spotting patterns. Explain that some brains are good at recognizing social signals, and some are not. And that we need all these kinds of brains.
A good example is computers too. If he knows about the differences between say a mac and a pc, or you could use video gaming systems. Then you explain how mac is good for some things that pc is not as good for, and vise versa. Or some video game systems are good for one type game, but not others. And it just depends which game (or program) you want to play or run. Both are good for what you need them for.
And I never called it a disability, I called it a brain difference. Though now in his early 20's, he knows it is technically a disability, and that it's ok.
Then, it's good to let him ask questions and answer as honestly as possible, and tell him honestly if you don't know the answer, and that the 2 of you could research together and find out.
I also told my son that he's just like everyone else in most ways, and can do most anything anyone else can.
Autism is just another type of brain, and may even be how the human brain is evolving now. He should never think of it as something shameful. There are so many amazing people out there on the spectrum who really have helped advance us in our modern world. I highly recommend Neurotribes (book) for you. It tells a complete history of autism, and tells of how autistic people have contributed so much. Without them, we wouldn't have these computers, phones, even radio.
I wish him the best and hope you have your talk soon, and hope he will feel proud of his unique brain!

There are many good books about that written for children.