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I agree that it's very important to get your father to a doctor, but not just any doctor.  This is a very challenging situation, and MANY well-meaning doctors have no idea how to handle it.  So the first task is to find the right doctor: a geriatrician, a geriatric psychiatrist, a geriatric center at a good hospital; someone who is thoroughly versed and experienced with the various forms of dementia.  Once you've found that rare individual, meet with her/him and strategize on how to get your father in to see him/her.  Do whatever you need to to "trick" your father into going.  The goal is not to convince your father there's something wrong with him, but to get as clear an idea as possible what is causing this change in behavior and what can be done to mitigate it.  The right doctor will be able to at least rule some things out, and come up with a likely diagnosis and treatment plan, and will be able to work with you to come up with a way to implement the treatment.  Don't expect your father to even be part of the discussion.  From what you've said, he's not just unwilling to see there's anything "wrong", he incapable of understanding his situation.  Trying to "get through to him" will not work.  In fact, it will almost surely make things more difficult.  You and your mother (and mostly you, probably) must work with the doctor to find a way forward.   Good luck!  And do contact your local chapter of the Alzheimer's Association.  They have many valuable resources.  You'll need all the help and support you can get.