How can HE get custody?

My boyfriends FATHER has a 11 y/o daughter who he didnt know about until she was 4, therefore his name isnt on the birth certificate BUT her. mither took bim to court and got a paternity test done so she could force him to pay child support (she lives off the child support he pays). The little girl no longer wants to live w her mother and wants to move from Iowa to GA to live w her father BUT HIS NAME ISNT ON THE BIRTH CERTIFICATE, so he says he legally doesnt have any rights to her. Ive told him that since he has proof that hez her father (lab dna test) and he can prove that her mother is unfit for her (she does meth and runs around w strange men, constantly bouncing from one house to another) he should be able to get custody. His daughter called todayto wish her daddy a happy birthday and was crying saying they were abiut to move again and she wants to come down here to see him and never go back home. Do any of you have advice that i can pass along to him?