Adoption of a child (born overseas) by a USC does not bring with it an automatic right to before USC.
The process of bringing an adopted child to the US seems to depend on which path of adoption has been followed and whether the adoption is finalised in the US or overseas.
There is just not enough information in your post to know which way the adoption is being completed.
My “guess” is that unless it is being arranged by an authorised adoption agency following the Hague adoption process, that they will need to need the “Immediate relative process” which can take years and has significant physical custody requirements ( ie parent must live with the child outside the US). Also, the USCIS link identifies some issues with this process when adopting children from Hague convention countries but not through the Hague Process.
I hope your nephew is fully researching this prior to any adoption, as he may unfortunately find himself with 2 adopted children who cannot enter the US to live for several years or worst case , cannot enter at all. He needs an adoption immigration specialist before he adopts anyone.