He doesn’t have to agree, if the government grants it, it’s a done deal, this is how they did with mobster Giancana in the 70’s and the doctor can refuse to testify and he will be held with contempt of court.
The courts have rule against doctors privilege, it is rare but it has happened. Two such cases:
- People v. Wharton (1984, New York)
A doctor was nearly held in contempt for refusing to testify about a patient who had confessed to a crime during treatment. The court ruled that doctor-patient privilege did not apply because it was relevant to a criminal investigation.
- State v. Betts (Tennessee, 1991)
A psychiatrist was held in contempt for refusing to release mental health records in a homicide case. The court ruled that the need for justice overrode the privilege.
So it can happen, and there are other cases. But, government can file contempt charges and turn him over to the DOJ.