Need Custody of first child

Myself and wife are both doctors. Both worked in Delhi. After the delivery of first child, we moved to Port Blair, Andaman to work there. But I had to resign from the job to take of the baby who was 9 months at the time since my wife earned more than me. She gradually developed illicit relationship with a man unknown to me and got pregnant with his child which she kept a secret from me for 9 months until the delivery of second baby. On the day of delivery I found out that the child was not mine, and she too confessed. She begged me to stay at least for the first child, and also offered me to take the first child if I do not wish to stay. I was in state of shock and mental trauma. Seeing the face of my first child (son) who was just 2.5 years at that time and how he can suffer bcoz of the broken family, I decided to stay and decided to not tell anyone. But, everyday this incident was killing me mentally. Finally, after 9 months since the incident I made my mind to get separated. I want to apply for divorce on grounds of illegitimate relationship which I can prove easily. All I want is the custody of my first child. Please help me.