i can tell you from experience that autism does not care a lot about exposure. if you're overstimulated you're overstimulated. and also it's often much worse as a kid/teen. maybe try exposure therapy again when they're a more mellow adult. when they've learned about their own condition and can logic their way into a solution, instead of misery being forced on them by people who don't understand

Translation: being forced to suffer quietly and pretend it's okay makes it harder to actually ask for help when you need it, and that's super convenient for people around you so you should do that.

(The one moderated comment in the OP said the same thing btw)

If you stab yourself with a small knife, in a few weeks you will be able to stab yourself with a larger knife. Continue doing this and you will become immune to being stabbed.