Where does your father live and how large/small of a man is he? I ask because it will greatly dictate what size firearm you carry. I live in the Panhandle of Florida and there is no way in hell I'm going to be carrying around a big 44 mag revolver and actually conceal it comfortably, even being 6'2" and weighing about 270 pounds.
and he would like to get a .45..
Your father needs to worry far less about the cartridge/caliber and more about getting a comfortable firearm that he will actually carry. It does him absolutely no good to get a 1911, a big ass N frame revolver, or other full size firearm if he's just going to leave it in the safe because it is too cumbersome and uncomfortable to carry every day. What other people carry means absolutely nothing when they aren't going the be the person carrying the firearm.
so just to ask what everyone else uses. and what is easiest to carry.
What's easiest to carry is going to be smaller framed revolvers and slim, single stack automatic pistols. Such as S&W J frame revolvers, S&W shield, Springfield XDS, Glock 36, Kahr PM series, Ruger LC9, Beretta NANO, Para Ordnance LDA,
I personally carry a Smith and Wesson model 442. It's a small alloy framed, 5 shot, 38 special revolver that is extremely light weight, and is very easy to conceal in shorts and a T-shirt.
Bottom line is that if your father, or any of us for that matter, have to use a firearm to defend ourselves it isn't going to be from across the street. It will be up close, 10 - 20 feet, and any cartridge from the little .380acp on up to .45acp and larger.... the dead guy will never know the difference.... assuming you can put the bullet where it is suppose to go. If he can't, well, misses don't amount to much no matter how big the bullet is.
Another thing to keep in mind is that the larger the cartridge the greater the recoil and muzzle rise and the longer it takes to reacquire your target and make a second accurate shot all things being equal.