Dear! I would suggest you to take him to the eye specialists as he can diagnose what's going wrong with your father's eye, do visit soon as soon as possible.
Sudden eye turning in adults start with double vision first. Brain due to this double vision find 2 images and try to adjust the eyes and as a result eye starts to turn inward or outward. Another reason of eye turning in adults is stroke. There is no direct evidence of dementia and eye turning but as dementia affect the brain and can cause hearing and vision problems. So, discuss this with your doctor.