My son has turned on me and started critiquing me even though he knows I am sensitive. Why does he do that?

Power play. Sounds like your son is into oppression. Some say motherhood is a form of oppression. He is taking advantage of your peaceful, nuturing and tolerant behavior. Time to take a stand and be strong and the Alpha mom! Time to send him off to boarding school? (joke).
He learned to. Did he learn it from dad, or is this something he picked up outside the home. I would find a family therapist and go yourself if no one with you. You need help developing the skill set to work through this.