Usually, people improve their walking with rehab and exercise after a stroke. His issues with walking may be due to other health problems, like arthritis or dementia. He may just
need an assistive device, like a walker or a cane. It is also entirely possible that he had another stroke that has made his walking worse again. My advice would be to take him to his doctor to see if they can figure out what is making this happen. Perhaps it something that can be helped, and his walking might improve. They can even order some physical therapy, which might help him walk better or get him assistive devices that may make him more sure of himself.