I doubt a doctor would really know that much. Doctors are great at generalisation on many things... but probably not on something this specific. Since Avodart was created as a prostate medication initially I would imagine it will keep your prostate a relatively unexceptional size, as intended.
I'm assuming jambri answered in view of what your local doctor, your GP, would know. Something as particular as this would really require a specialist's opinion - whatever the speciality dealing with all things prostate is.
Unless a doctor specializes in "Hairloss", they would probably know just about the same, or even less. Even doctors don't have the time to keep up with every aspect of the human body, while many of us have spent countless hours researching hairloss.