Suggest Treatment For Bone Loss In Knee Joint After A Fracture

Hi doctor, My father is 62 now. He had a fracture in foot 3 years back. Now he is suffering from severe knee pain, in both the Joints. Doctors are saying that, it because of bone loss in the knee joint and asking him to do some simple exercises. He is following that now. Is there any permanent cure for this bone loss. He is unable to sit down and unable to bend his legs. He some times finding it very difficult to get in to car with very less height.
Please advice.