The best place to start is with his family. Absent that if you know her name and where she was born I can give you a list of tribes to contact. You might want to try an online database search. There is usually a fee for these services. You may want to pursue this in family court and compel disclosure. Good luck.
This is not intended as legal advice. It is only provided for educational purposes and cannot be relied upon as legal advice. Further no attorney client relationship is or has been formed by answering this question.