If your dad is touching Rez, then weight and ease of cocking may have to play a part in this situation. Also ease of actually loading the pellet and .22 might be better for him to hold / load.
My shooting friend of many years is 81 in a couple of weeks although you would place him in his early 70's so we are not aging the same, I am a perfect example, if you saw me, well you might place me in my late 30's............................or not. The point I am making is buy a rifle which not only suits you but one he can actually use easily.
Phil