Consult a physiotherapist... Get it assessed, diagnosed and get the treatment.. It can be a nerve compression/muscular spasm or venous problem.. So its necessary to get the right assessment.

Do stretching exercises before walk and after walk do hot fermentation

Calf Stretches Standing calf stretch Stand away from a wall and.put your fight foot behind you and be sure your toes are facing forward. Lean forward at the ankle while bending the right knee and keeping your heel on the ground. Hold this for 30 to 60 seconds. Repeat 10 times a twice a day. Downward dog yoga stretch Get down on all fours with your hands under your shoulders on the floor. Walk your hands forward slightly on the floor. Spread your fingers apart to allow for a broad base of support. Push your hips up toward the ceiling and tighten your abdominal muscles. Keep your heels on the ground and gently try and straighten your knees. Repeat 10 times a twice a day.

he may have nerve compression on lower back. consult physio to get proper treatment.

Get assessed  with physical therapist or general physician to have clear idea whether is due to musculature or circulatory  cause.

Seems like neurogenic claudication commonest reason being spinal canal stenosis could be because of facet hypertrophy again very much common in old age people... Kindly consult a pain physician ... Let him evaluate u properly before coming to any conclusion....

Hello....... Deep vein thrombosis or any other vascular anomaly could be the cause.. Or it can also be muscle cramp due to fatiguability, take him to general physician once. Electrolyte imbalance can cause muscle spasm sometime... Tell him to move his legs while sitting to establish blood circulation.