Clydefrog beat me to it, ever got pushed onto your back? For me it's been a while and I'm a bit heavier now. It would only hurt worse. Since then the only time I've fallen on my back has been when slipping on ice or a steep slope.
I spoke to a man in a pain management clinic (therapy, scans, rods, meds) who hurt his lower back while inner tubing down a shallow river. He slipped off his inner tube and his back hit a rock at the bottom. The pain will be with him the test of his life. He can't do what he used to. I thought, "man what are the odds that the rock hit dead center on his spine? On vacation too" Just a couple inches over and I think it would have been different.
That being said if he must try for novelty there are the hybrid IWB holsters. I have a Theiss holster (also see Crossbreed, Old faithful, comp-tac, desantis king tuck) which is really made for 4:30 and farther to the rear rather than a3:00 strong side. Looking at the design, the widely placed clips and wide slab of leather really distributes weight out well but the width would also rub your leg if you wear 3:00 when you bring your knee up as you walk or sit. I can slide it over as much as I want - even to 6:00 if I wanted to and I'm sure it would feel great as I walk but I find that 5:00 or 4:30 accomplish the same without placing the slide against my spine. The body has a natural curve/hollow anywhere above the rear end so it conceals just as well - in my opinion. If your dad hasn't tried carrying this way I think he should give it a go at some point. This style of carry is versatile and a adjustable - you can set the overall height of the gun and set the cant/tilt by adjusting the clip height. Gun stuff for Father's Day? What a great family