Becoming Legal Guardian of child

My ex wife has a son that i raised since birth. When we got divorced there is nothing about child in divorce since we both knew he was not biologically mine. Since divorce she has given him to me to care for. She does not want to sign over legal rights to him whatsoever. But does not want burden and expense of child right now. He currently lives with me my wife and two other children. I am looking for some document or court paperwork to fill out to become a legal guardian so i can enroll him into medical coverage as well as school and all other parental things. I do not want take him from her but need parental rights so i can care for him properly.