my husband cheated on me, having affair with her office colleague

After six years of married life & 4 years live in relationship before marriage and having a child of 5 years old, he cheated on me, having extra marital relationship including s*xual relationship with his office colleague. After he got caught by me and many other people... now he is saying he had done a mistake & i am responsible for that as i am doing one extremely busy job. Now he is claiming he has stopped having relationship with that woman. But on & often he becomes violent & starts me abusing & beating me physically. I am in an extremely helo less situation. I was Muslim & he is hindu by religion & got married under special marriage act. Now i want him to become loyal & committed as my sons future is involved. My 5 years old son is in an extremely bad mental condition as on & often he is seeing us quarrelling & my husband is beating me. I want to continue this marriage for the sake of my son, otherwise he has to spent his life without the guidance of his father. My husband is alcoholic too. How can I take the helo of law to make him committed & to stop violence & physical abuse on me. Please help me.