How Can Stabbing Foot Pain That Is Intermittent Be Treated?

My father is an 88 year old diabetic. He is fit and has his condition under control. Recently, he complains of foot pain - sharp, stabbing ONLY in his left foot. He moves his toes rigorously without issue...poking the external skin does not create pain...cannot identify exactly the source. Soaked in epsom salts...no relief. Comes and goes. Unpredictable, but, limited one area of one foot.