S.T.
I think you should end it!!!! it is very possible that the time you are away and he is alone with her she is being mean/rude/abusive with him. kids act like that usually when something is wrong, she may have scared him into not telling. I had a babysitter when I was that age who was very mean and distant. she watched one other child along with me. she would make me clean up her house while she sat and watched tv. she made me go down to the front entrance, she lived in an old weird style apartment building, and unlock the door so the mailman could get in. if i didnt do it right or if for any other reason she decided, she would make me sit on the couch facing the wall the whole day, not even let me have lunch. i didnt tell my mom at first because she always told me my mom would be so mad at me for "being bad". I say fire this K and keep your son safe and happy.