How do I tell my mom that I don't want to talk to her very often anymore?

My parents are in deep denial over my son's special needs (low functioning autism, non-verbal and mental retardation) EVERY time they see him (thankfully isn't often for the live in another state) the conversation consists of "well....THAT looks normal!" (he can be watching a tv show, he is a person after all!!) There is a lot more to it but I am to the point (and with the advice of a therapist) to cut their involvement out of my life to a minimum. I don't know how to tell my mom "I wont be calling you every day, probably only once a week because that is all I can handle of you" She WILL ask. How do I tell her that it is her attitude towards my son and her denial that led to this?