Look closely at his diet. Also he may need to be detoxified. This happens frequently that poisons in the body when circulation gets bad that legions appear on the skin. Get a good book on detoxification this might be the problem.
I don't know what the sores look like, but has he been checked for shingles? When people have had chicken pox in their young years, sometimes the virus recurs as shingles later on in life. It is terribly painful, and produces sores. I would assume that his doctor checked or that the sores don't look like shingles sores, but it is worth asking.
Here are symptoms of diabetes:
Frequent urination
Excessive thirst
Extreme hunger
Unusual weight loss
Increased fatigue
Irritability
Blurry vision
Did they take a Fasting Plasma Glucose Test (FPG) or an Oral Glucose Tolerance Test (OGTT) at the doctor? That would be the test to determine if your father has diabetes and at his age it would most likely be Type 2 Diabetes. Also you could go the American Diabetes Association Website http://www.diabetes.org/diabetes-symptoms.jsp. There is a lot of useful information on that site and a lot of information about Type 2 Diabetes.