S.T.
assuming that YOU are telling the truth in this post, i suggest cutting your heartbroken, confused and lashing out small son some slack.
his world is being torn apart, and you're worried more about your image than the damage being done to your little boy.
continue to maintain your stoic silence regarding any negative talk about your ex. stop trying to 'convince' her. if she knew you, she'd know, and she would understand what's going on in your son's world.
if she really believes you're trash-talking, maybe she has good reason.
stop trying to 'get to the bottom' of your son's words and start focusing on being a good and supportive parent to him while BOTH his parents work on making this divorce as gentle as possible on him.
khairete
S.