bipolar.. kid with bipolar threatning to kill me.?

my wifes son. ( my step son )) is 13 and when he gets mad at us for telling him to to clean his room , take a shower, brush his teeth etc. he throws a fit . i do not get involved untill he starts yelling at his mom or cussing her. and when i do get in betwen it he threatens to stab me in the neck when i go to sleep.. we have on numerous occasions knives under his bed and hid through out his room. my question is just what measures can i take when he threatens me . he is his moms lil boy and she constanly makes excuses for him and rarely takes up for me . i want to get the police involved so if he ever does come at me with a knife his actions will be documented . i live in the state of ga. so i know laws prob differ from state to state . he has a dr. that he sees once a month . but seems the older he gets the worse he becomes.