Legal and Formal Child Custody papers

My sister was married in 1998 and had a child soon after. Issues in the marriage started immediately after birth of the child and she and her child came back and started staying with me and my parents. Her husband applied for ex parte divorce on grounds of desertion, abandoning and cruelty and the same was granted in 2005-06. We are in possession of a copy of these papers. However, these ex-parte decree papers are silent in terms of child custody. The child continues to live with us and we have been bringing him up and financing his education. At this point, 10 yrs after the divorce finalization, do we need to get any formal or legal document, which states that the child by virtue of living with his mother, grandparents and uncle (myself) for all his 17 yrs of age has been in full custody of his mother alone ? We are planning to also change the child's surname to our family surname through court of law For this and similar purposes in future, will we require some formal or legal document declaring that child is in the custody of mother?