Want to get the custody of my 8 years old son without divorce from her

I am working as a researcher in premier institute in India after finished my Ph.D. in Engineering from the same. I found evidence of adultery of my wife. She is diploma in Engineering. We live separate since 4 months. I have not the access of my child from this date. I want to build my son better future by personally teaching him. I also taught him when I live with him. I have clear plan for his better education by teaching him daily. I did not file a divorce petition till now as I belong to Schedule Tribe and Hindu Marriage Act is not applicable to us. We have our customs to take the Divorce. However, I want the custody of my child as soon as possible to help him for better education. What are my chances to get the custody with this circumstances? What should be the immediate steps from now to meet him.