Indonesian citizenship is exclusive. If his stepfather took out citizenship papers for him, he had to renaturalize on reentering the U.S. Did he do that? Did his mother do that for him? Remember, she was supposedly an Indonesian citizen herself when she died.
I’m not sure about that... I thought that the citizenship granted by Indonesia to children of non-Indonesian parents was conditional until the child was 18. And the US does not allow you to forsake your citizenship until you are of legal age (18), meaning your parents cannot force you to lose your citizenship.