Invalid marriage without physical contact for 5 yrs & no maintenance payments?” (79 characters)

I am a 36 year old married woman, I was married to my khala’s son, after 10 months we had a son, who is 6 years 10 months now. But after marriage my husband used to visit us every year for 15 days for 2 years. He even took me to Qatar, but after coming from there, he did not visit India for 2 years, then he came for 3 days just to divorce me. But all matter settled down and divorce did not happen. But from past 5 years, we do not have any physical contact with each other, no intercourse, and he is also not paying any maintenance to me and my child. In the meanwhile, as I was staying with my in laws, my brother in law took my nude clippings by fixing camera into my bed room and bath room and started black mailing me. And neither they are divorcing me nor giving any maintenance to me and my child. They are asking me to give khula because they want to save the mahr amount and take custody of jewellary and my sons.
I heard that, if the husband and wife are not having sexual intercourse for 6 months, the marriage is said to be in valid. Please advice me what to do, because I do not want to give khula, and custody of my son.
Please also inform if (1) My marriage is valid or not? (2) Is there any necessity to sign on paper for divorce?