you need to challenge your DS’ beliefs, or this isn’t going to get any better.
Sometimes you can sometimes you can't. Even when you can,it doesn't happen overnight or simply by explaining something.
To the kid, the teacher lied and her good intentions don't matter, because lying is wrong and he didn't trust her to begin with. In his eyes lying just reinforced that.
PP can talk about good intentions until she's blue in the face, the most likely outcomes will be it either goes over his head or he starts to lose trust in mum as well.
Don't forget he's only 6, few kids have the emotional intelligence to differentiate between lying is wrong and she had good intentions,poorly executed. Especially given the fact that he has ASD,trust issues and rigid thinking.
You can rationalise it as an adult and a teacher, he won't be able to.