His autism is irrelevant when you made your boundaries clear and he uses it to brush them off.

His autism is not an excuse for crossing other peoples boundaries. I have autism myself and I try to stay within other peoples boundaries as long as the people are clear as to what their boundaries are I’m fine with it.

That's a good line! I'm saving that one for when my flirting goes awry.

I'm pretty sure your have to date him or is discrimination.

And my autism is making me tell you to deck him next time you see him, because autism doesn’t work that way.

I'm autistic. This is not how it works. However, I will say I'm sure at the time he thought it would sound hopeful and maybe even romantic. God knows how often I've started fights with my partner over something that came out wildly different from how I meant it. How recently did he become an ex? Autistic people struggle with change. If it was very recently, he's used to you being in his life and is unwilling to let the idea of being in a relationship with you go. It sounds like you're already making your boundaries explicitly clear, which is good and what autistic people tend to need since reading subtleties is difficult, but keep making them firm and telling him in no uncertain terms that it's over, period, end of story, have a nice day. It might take him awhile to come to terms with the change before the grieving process even starts and you'll probably have to distance from him while it's happening, but it'll happen. If it was not recently, like a few months or longer, you might just have to ghost him if you can. He's not getting the message.