Since the gun is to be used for home defense instead of concealed carry, I really see no advantage to a small handgun. A full-sized semi-auto or revolver will have more weight to soak up recoil, a longer barrel to give higher velocity, and a longer sight radius to make accurate shooting easier. Also, small handguns often have longer, heavier triggers than their fullsize counterparts for a variety of reasons.
If he prefers a revolver, a medium frame gun in .38 Special or larger (depending on what level of recoil he can handle) with a 3" or longer barrel would probably be the best choice. Good guns that fit this category include S&W K-Frames like the Models 10, 13, 15, 19, 64, 65, 66, and 67, S&W L-Frames like the Models 581, 586, 619, 620, 681, and 686, the Ruger "Six Series" revolvers including the Security Six, Police Service Six, and Speed Six, and the Ruger GP100. Colt has also made some good medium-frame revolvers including the Police Positive Special, Official Police, Lawman, Trooper, Python, and King Cobra, but Colt DA revolvers are not longer made, have limited factory support, and relatively few gunsmiths are familiar with them.
If a semi-auto is more to his liking, a full-sized gun (4" barrel or longer with a full-sized grip) in 9mm or larger (again depending on what level of recoil he can handle) would probably be best. While polymer guns are nice and have certain advantages, the extra weight of a metal frame would be advantageous in this particular case because it will help to dampen recoil. Good semi-autos to consider include 1911's, Sig P220-series guns (220, 225, 226, 228, and 229), Beretta 92/96 series, CZ-75/97, S&W 3rd Generation autos, Browning Hi-Power, or even the Walther P38/P5.
Other factors that should be considered are the fit of the gun's grip to your father's hands, what sort of sights he can use easiest, availability of ammunition in his area, complexity of the manual-of-arms/ease of use, the level of recoil he can handle, and of course budget.
If he decides that a long gun would be a better option, you've got a few choices to make, but since you posted this in the handgun forum we can only presume that a handgun is his/your preference.