Can I sponsor my adult who nephew is currently studying on an international visa?

Regrettably, there is no visa category by which a U.S. Citizen or a Lawful Permanent Resident (a "Green Card" holder) may petition for a nephew. It would be wise for your nephew to consult with an immigration attorney to explore any other eligibilities, options and strategies for his immigration-related goals. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.

Not through family immigration.