Hello..your father is having claudication pain due to Lumbar canal stenosis. He walks for some distance and then the pain in the lower limb increases, he has to sit for few minutes and can walk again for few distance..this is typical intermittent claudication seen in patients having lumbar canal stenosis. He must have been prescribed Pregablin medication.If that is not helping him, second line of management includes Epidural Steroid injection( But no cure guaranteed-still one can try if they want to avoid surgery)Lastly Surgery will definitely help him to relieve his symptoms. Hope he gets well soon