My sons father needs to stay away and I don't know how to do it! Please someone help!?

I stayed with a man for 3 years caz he had a drug problem that he promised me would go away. I tried to help him again and again. Now for the past 2 years we have had a son. Who he is not a part of. He is not dependable deals drugs drove in car wit drugs and my son smokes pot everyday lots of times caz it keeps him calm has disappeared 8 times this year from coke does nothin wit his family or extended family(birthdays dinners favors etc) but does everything wit his friends. Has not had a real job since i meet him. God I could go on and on and on But the deal breaker here is he spit on me shoved me twisted my banged my head on window all in a moving vehicle with my son in car on vacation caz he did coke the night before. I live off his mom (so does he) have no car no job and dont know where to start. I dont want to let him see him til he gets help but man is his mom frsking out yeah not him but his mom! She has money she has gotten him outta alot of situations she is his biggest enabler. PLEASE HELP! Can they take him? Can he really get joint custody? I just want him to go away!