Have a heart-to-heart discussion with your dad about your concerns. Get support from other family or friends who can join you in having a serious, yet supportive conversation with him
The reality is your father may refuse. Sometimes it takes an unpleasant medical issue to surface in order to turn stubbornness into a learning opportunity. In the diabetes education world, this is known as a teachable moment. People who have been in denial about their disease for years get hospitalized with a diabetes-related complication, see the light, and then turn their lives around.
The second issue you raise is an age-old one. Unfortunately, most people don't have the discipline to eat healthily or exercise on a regular basis. In your father's case, he could feel unmotivated as a result of high glucose levels. Does he have a glucose monitor and know how to use it? Are you setting an example by eating healthily and getting exercise and encouraging your father to join you in these activities?
To find lots of other ideas about how to help your parent eat well and get moving check out these two articles on diet and exercise.
about your observations of his changing health status. Ask how your father feels. Tell him you are worried and you'd feel better if he saw the doctor for a check up. Why not invite him to join you in attending a local diabetes support group as a compromise?