It depends on what he is asked to testfy about. For example, if he were required to testify about his citizenship, he would be compelled to answer. However, if he were asked to testify about matters that could be considered national security matters, the answer is most likely no. The exception would be if the courtroom were closed to the public and ONLY the Judge, Prosecutor, Defending Atorney and (maybe) the Jury could be present.
Wow that could be get VERY interesting........ discovery.