How should I go about searching this situation for my family tree?

Okay, so I am currently in the middle of researching my family tree. I interviewed my mother to get information about any/all family information that she knows. She revealed to me that my second great grandfather was raised by the Fields(my family name), but was not born a Fields. From what she knows his last name was Elie(we don’t quite know the spelling). Apparently, he was orphaned really young. All of the documents that I find(death certificate/tomb stone/etc) show him as a Fields, because like mentioned he was raised from a young age. Since he is in the direct line of my ancestors, I want to find his biological parents. I don’t know how I would go about doing this, since he was not “legally” adopted.