How can I take custody for my child from my husband

on 10 November 2016 Dear experts, m mus woman stay in Pune with joint family. My husband visit Vietnam for business since 5 years . The gal who takes care of office in Vietnam is my husband's partner in business . he has told tht gal tht I hav no relation wit my wife, she is only for world I sleep separate in my room and she wit child in other room, and this is true as we 3 can't fit in one bed and we hav big house so I used to sleep in other room which next to my husband room , he is blindly in love with tht gal and he does wht she tells him. before 5 years I couldn't take n e action as my son was small and joint family I hav so didn't know where to go and wht to do ? Whom to contact, My husband brain is very powerful . I want my son custody and he knows I bare this affair just bcoz of my son. I don't want my son to suffer as he doesn't take care of his own paralytic mom and unmarried sister.. my sis in law takes Care of my mother in law and