My Friend Is A 68 Year Old Male Who Has

My friend is a 68 year old male who has rather rapidly lost his mobility. In June he was able to jump into the lake from my pontoon and swim and now he can barely walk. He can barely lift his feet enough to walk up the stairs and says that his legs feel very heavy. He also fell several times and had difficulty getting back up. He was seen first by an orthopedic surgeon who ruled out any joint issues and was referred to a neurologist. His examination was followed by a blood work up which was negative. He ordered a MRI of the spine which was negative and now my friend is scheduled to receive a MRI or some kind of brain scan. There does not seem to be any amount of loss of cognition or speech, vision, memory or anything other than mobility, walking, getting up from a chair without the help of a walker. He uses a cane outside and a walker inside his home but probably should just use the walker. needless to say we are very concerned. He has no pain except a recent extreme back pain that was treated in the ER and has resolved with pain and anti inflammatory meds. They said it was muscle spasm. There is no history of any trauma or strain. What might the brain scan show from a good to worse prognosis. Thank you very much for your consideration and expertise.