Since the guy with the locked iPhone already plead guilty or whatever, can't they give him immunity for anything new and force him to unlock his phone that way?

Since the guy with the locked iPhone already plead guilty or whatever, can't they give him immunity for anything new and force him to unlock his phone that way? I'm no lawyer but as I understand it, that's how prosecutors can generally go about forcing people to give evidence about co-conspirators. From there it's a straightforward contempt charge and the guy sits in jail until he gives up the passcode.