Calf pain can be caused by many things, but if the pain happens at rest and has been going on for this long, it may be Peripheral Arterial Disease. It's fairly common in men over 50 (you didn't mention his age) and particularly among smokers and diabetics. Given what you've said here, he should definitely go see his doctor about this.
Are you on any medication? some meds. will cause muscle pain.
He should be seen by his doctor.
potassium deficiency may cause cramps. potassium rich foods, including
Bananas, apricots, nectarines, dates, grapes, or raisins, Beans, Cabbage/broccoli, Oranges, grapefruit, and their juice, Pork, lamb,
Potatoes, corn, Saltwater fish ( tuna), Tomatoes, tomato juice.
Make sure you drink plenty of water during the day.
Apply a hot compress to the cramped muscles.
Stretch and flex the muscle Lie on your back in bed, lift your legs, extend them toward the ceiling, and then point your toes to the ceiling and flex them slowly back toward your calf. Repeat this until the cramps stop. For a good stretch, grab your toes while still in this position and pull them down toward your knees. You'll feel this stretch working in the back of the calf muscles.