Subpoena him. Put him in a public hearing. Make the rat take the 5th like a communist in the 1950s.
But reading FindLaw on the fifth just now sheds a different light on it:
At trial, the Fifth Amendment gives a criminal defendant the right not to testify. This means that the prosecutor, the judge, and even the defendant’s own lawyer cannot force the defendant to take the witness stand against his or her will. However, a defendant who does choose to testify cannot choose to answer some questions but not others. Once the defendant takes the witness stand, this particular Fifth Amendment right is considered waived throughout the trial.See "Fifth Amendment Right Against Self-Incrimination"