Suggest Treatment For Pain In The Knees While Walking Or Running

Hi! I have some questions that I hope you might help me answer. My father has had a problem in the last couple of months with his knees. Whenever he hits his heel whether he is running or walking, he experiences a ton of pain in his knees. Whenever he runs on his toes, like when playing tennis, it does not give him any of the pain that he gets when he strikes his heel. He went to get an X-ray, and that doctor said that his bones and his joints look better than expected for a guy that runs the amount that he does. Do you have any idea how to help us?