I agree with "NOW."
My husband was adopted, and he says that he's pretty sure his mother whispered, "You were adopted," to him in the cradle.
It was never a big deal to him because it was just something he always knew.
I think this is similar. Not talking about it and waiting for some big reveal gives it a weight that might make it seem very negative to him. People don't usually keep good news a secret, and you're probably not planning on eventually having a "Surprise! You're autistic!" party.
Telling him simply now seems better to me, maybe even a casual, "We're both a little weird because we're autistic," the next time he calls you weird, followed by mentioning that that means our brains work differently than those of most people. He doesn't exactly need brain diagrams and discussion about his amygdala just yet, just a recognition that he's different and this is why.
I wish I'd known why I was different when I was in school. I think the difference in positive vs negative is in how you treat it. If it's treated as a positive, exploring his strengths because of it, or at least as just a neutral fact, then it should be easier for him.
Last point: He's more likely to come ask questions if there's already an open dialogue between y'all about this. For many of us who didn't know until we we're adults, our differences were viewed as a defect, which made the idea of asking someone about them way more difficult than it had to be.