She said she can't term because she isn't the boss.
Op~ DO you have a way for mom or your boss or both to possibly observe this child while not being seen by him? Would the mom perhaps give permission for him to be video taped so you can show both her and your boss the level of seriousness you are trying to convey to them?
Maybe you could also have other parents who have witnessed his behavior speak to the director. If i were a parent seeing this when picking up my child, I would be terrified to leave my child there...kwim?
I would also check and see if you could maybe have a behavioral specialist come in and observe so you can get a professional opinion.
FWIW~ I know EXACTLY how you feel about not wanting to come to work because of one child who can show the people he chooses that he is capable of behaving just to be the opposite when he thinks no one is watching. (((hugs))) simply for having ot go through this.