My 11 year old cat has started hissing at our 3 1/2 year old son. We don't understand because she was here before he was born. However, he is Autistic and has just started to acknowledge her. I am present when he reaches out to touch her. She doesn't even let him extend his finger and she hisses loudly. He then cries because he knows it is an act of agression. Why? And more importantly how can I get her to stop?