If the both of you are happy with the current status of the relationship, I'd not bother to say anything more about it. You're both adults, you can choose to ignore your parents outright. Frankly, it's none of their business.

That said, I can understand that they are only trying to help you, want the best for their daughter, etc. I'm not saying they are but, perhaps they think him less human for being autistic on some level. Like, he'll never be good enough for our daughter or something. (It's not coming through like I want it to via text, I know. Note the fathers reaction, in the movie "Adam", to his daughter dating an aspie).

Being an aspie myself, and feeling more or less the same as your SO about healthcare/mental health "industry" in general, this would cause me no end of stress and anxiety. Having been though somewhat similar life experiences as you describe he has, pressing me into a situation I've already stated I don't want to be in would also start feeding feelings of anger and resentment toward those doing the pressing.

Those feelings would lessen in time, but they'd always be gnawing at the back of my mind every time I encountered your parents, for example. I'd probably do it for you, just to make you happy and so you wouldn't leave me but, the same feelings would apply to a lesser degree.

I'm sorry if something came out wrong, it's not my intention to sound like I'm talking bad about you, your parents or your SO. I'm simply trying to offer some insight on how'd I'd react in his situation.

The long and short of it is that aspies, (at least myself and most others with an ASD I've encountered do), seem to like to take things literally and to heart. Keep this in mind when interacting with your SO.

I hope this helps you somewhat. :/