It's likely there's a loss of blood supply to the foot on that side. This could be a vascular (blood vessel) problem. It could be caused by blockage of the blood vessels in the calf just above the foot. Atherosclerosis plaques and/or blood clots are the most common vascular causes of temperature and color changes in the foot.
Another possible cause of these symptoms is a tumor pressing on the spinal nerve to the foot. No matter what is causing this problem, a visit to the doctor is in order. Loss of blood supply to the foot can cause major problems. The earlier the problem is diagnosed, the better the results. If he has a primary care physician, call the doctor's office right away. If he doesn't have a doctor then call the emergency department, explain what's happening, and follow their directions.