Leg pain , at his age is often associated with a nerve getting pinched in the spine. Do not get worried , this is very easily manageable. Unfortunately an X-ray does not help detect this , as an X-ray is done to look at the spinal alignment and bones but this is a issue with soft tissue (nerves). If his pain has been persisting and getting worse , I would suggest you to do an MRI of the lumbar spine to confirm the suspicion. Once we know the exact cause, it is much simpler to start a physiotherapy protocol in accordance with the pathology. This is a very common issue faced in old age and has simple solutions ! 80 percent of those suffering from this condition get better with a well structured physiotherapy protocol and a gel / tab gabapentin / hot compress and exercises. Please read my blog for the exercises that will benefit him. All the best !