How does this fear manifest itself? What symptoms are you seeing and when?
You can attempt to treat the symptoms or you can treat the cause. Whirly's ultimate solution was to treat the cause, which is ultimately more effective. But treating the symptoms could be a valid response with this person.
You could try breaking down the stall recovery into tiny actions and have the student concentrate on a specific step in the recover until they can perform it in a relaxed fashion. The criteria for progression should be relaxation. Emphasise this point to the student. At the very least you might determine that the student is afraid of some specific aircraft attitude or sensation, and this might for the basis for further work with them.
But your attempts to treat this so far (handsoff stalls etc) show you are already open to the idea of treating the symptoms based on the above approach, and they have not yet been successful.
Many mental blockages in training can be traced to a "command blockage". That is to say, the student is not "in command" when they say "I have control". i.e. they do not believe that they are in command. They still expect the instructor to be responsible for the ourcome of each exercise.
One way to undo this is have the student teach you. This would need to be setup on the ground, with the student having been given lots of time to prepare a "lesson". Give them time to talk to you as an instructor, so that you can confirm that they know all the steps to be gone through. Explain to them that you now know they had all the practise and have all the knowledge they need to perform the demonstration very well. Make sure they have made notes before they go off to prepare a lesson plan. Have them give you a ground briefing including a detailed description of the recovery actions, but don't critise any aspect of it. (You're not teaching them to be an instructor!).
This could work if it is the only problem the student has. It could also be combined into a pre-Check ride, so the student can practise a "Passenger" brief for the Examiner. This would again heighten the sensation that they are in command.
One final thought. You say the student has not have any bad experiences. If the student does not know he is afraid, maybe he also does not know he has had a bad experience!
Best wishes to you both.