Aldin,
From your previous posts I know you are the naturalized citizen and your family (wife and son) are having interview pretty soon. You do not need any custody papers for a child, if you were and still married to his mother. Your marriage certificate and child’s BS is enough.
Now, you never before mentioned you have two children….So, if this other child is not with the same woman (your current wife) then you should have a divorce, or child support papers from the court, certified by the clerk, showing with whom the child currently lives (the custodial parent) and how much of a child support you pay or receive every month, just like Cherr said.
And of course – if this child is a younger sibling of your 5-year-old and was born in US to the same parents (you and your wife…) – there are no any custody issues/papers you have to worry yourself about.
Good luck!