What Is Tiredness In Eyes Causing Dizziness Suggestive Of?

My father is 74 years old. He s in great health and looks like he s 50. The problem is the best year and a half he s been complaining about tired eyes which is making him dizzy most of the day. He has sleep apnea and uses a machine. He’s been to sleep studies, eye doctors, general practitioners and everybody has tried various medications and treatments but nothing has been narrowed down as to the actual cause. I feel like because of his age, doctors are just thinking it’s old age and prescribing him drugs such as early alzheimer pills or pill normally used for depression. He’s otherwise healthy and very mobile but because his eyes are always tired he is always tired and doesn’t feel like doing anything. Is there any specific questions I can ask the next doctor? I believe he is going to see a neurologist next.