Advises to get my 3 yrs Son's rights

Sir/Mam, I am from West Bengal married since 11 yrs but living separately for 3 yrs with my mother and my only son (3 yrs).I left my in husband's home due to his extra marritial affairs.When i heard and watch all this and came to know about his affairs i called up my in laws and my mother.But my in laws supported him in this matter telling that he is a male and males can do this activities ,its my fault that how can i handle this. We did love marriage on 2004 but against ours parents.So my in-laws didn't accept me in there family as daughter in law because they expect that there only son get marriage to a rich family so they get lots of dowry.I was start to living with my in laws but they didn't do good behavior with me .So after 1 yr my husband and i start living separately in out of West Bengal. There at first he (my husband) did good behavior with me .He do take care of me.I thought that we are living a happy married life.I was also start to do job there.But after 1 yr he start to do drinks at home also.He came home almost full of drunken and became fight with me ,hitting me dong things by force fully.But i never tell this things to his parents or my parents.As days passing he start doing regularly all this. Then i get pregnant and to give birth i came my in laws house.We have got a beautiful boy as our baby in 2013.i decided to go my husband home when baby become 6 months so i live in my in laws home.In this meantime i came to know about my husbands affairs and immediately went there with my mother.There i saw that girl is living in my home.Next day she left my home.Then i called my in laws and tell them the truth but they all ignored this.Then i left home and came back to my home town and start to live with my son and my mother. Since 3 yrs am living separately and looking after my son alone.My husband do not giving any expenses of son.When ever i say he said to give divorce after that he give only son's expenses. Now what will i do ? I want to give divorce but if he give my son's all educational and other expenses at a time cause for safe and secure of my son's future.But he do not agree to give at a time. So please help me by giving your valuable advises that how can i get my son's right for his good education and safe future.Eagerly awaiting for responses. Thanking You, Rima