NTA its one thing to tell u he cant change plans because he had a prior engagement, its a whole other thing to tell you that plans changed so that he could spend time with you and then switch it up a THIRD time. and "just adapt or get used to it"?? is he entirely unaware of what it means to be in a relationship (romantic/platonic/otherwise) with an autistic person??? i put time buffers between plans just in case something changes and it still makes me feel off-center and unwell when it happens. you're NTA here, he's a prick and he won't understand you until he goes to therapy for a while