Can somebody please tell me if I am able to file for custody of my son?

My son is going to be 10 years old. The adoptive parents are his grandmother from his fathers side and the step grandfather. Several years ago the adoptive parents split. He was adopted by them when he was around two years old. I had visitation with him on weekends until I moved from that city. Then I had some phone calls. He is now ten and has been asking questions about his mom..me. After I moved the grandmother took the role of the mother and had told him she was his momma. He finally asked and after feeling guilty she asked him who he thought was his momma. He knew it was me. I have other children and only left him and agreed to open adoption because during my difficult time he became stable there. He is now about to turn 10 and the grandmother has just finished rehab for the second time in his life. And has decided since he is having some issues to allow her son to raise him. Aallow me to say that his father was eager to remove the child support and not attend any parenting classes by signing his rights over about a year before I agreed to nopen adoption. Her reason is that she wants to be a grandmother now, and not a parent. he has asked her repeatedly when he gets to come home to her. He doesnt understand. I no longer get to talk to him or see him as I had before because my sons father doesnt want me too. The grandmother will not enforce him letting me talk to him or see him. She also has made it very clear that she has the paperwork that he belongs to her and that she wont sign him over to anyone. Can somebody please tell me if I am able to file for custody of my son.