Buzzcock #15 and twobit #20 said what I had composed.
But that was before I read your post that specified the home defense was the primary objective.
But first, I was going to ask what (action type, caliber) he used to shoot, what prompted him to shoot (work? self-protection, hobby?) and what prompted him to give it up. That information would influence my answers.
(Before concealed carry was ruled out)
Ruger has a 22 rimfire version of the LCR now. 22 Rimfire is not a particularly good self-defense round, but is cheap to practice with and, let's face it, self-defense shootings most of the time do not require all that much power. Bad guys, if they get challenged by an armed and resistive victim usually disengage after the first shot. I have been thinking about getting one of each caliber. 22 for practice and 38 (or 357) for carry. The LCR .357 is just so much lighter than my SP101s.
You are a good son. If you will be instructing your Father, getting him something with which you are unfamiliar might not be a good idea.
Short-barreld shotgun. If 12 gauge is too heavy, 16 or 20 gauge. A long gun and a cell phone are great for a stationary home defense plan. A 4" to 6" barreled 44 Special or 45 ACP or 45 Colt revolver can hardly be beat as a "nightstand handgun" Large, slow bullets do well on flesh and tend not to go through too many neighbors' walls. 5.5" Ruger Redhawk 44 magnum loaded with 44 Specials is nice. Smith & Wesson 6" Model 29 loaded with 44 Specials or a Model 25. If he favors semi-autos, the Taurus PT145 I own has proved reliable, accurate and easy to use. Springfield's XD, Smith's M&P and the Glocks have good reputations, but I have only ever shot the Springfield that belongs to my friend. And, of course, the venerable 1911 design available from Colt, Springfield, Smith, Dan Wesson, Kimber, et al.
Lost Sheep