Hi Sayyadina: does your dad have any other close relatives or friends? Does he happen to belong to any religion or church? Perhaps there are others that could share these concerns with him as well. An intervention of some sort?
Unfortunately though, one has to personally acknowledge and accept they have a problem and want to get help for this to work. I guess in the meantime, just letting him know you are around and that you care is a positive step in itself.
I guess the only exception is if you feel he is a danger to himself. Then you could perhaps talk to his doctor or take him to the ER for evaluation.
Hang in there. It sounds like he is very lucky to have such a concerned daughter.