I am by no means an expert on photo editing. Just tossing possibilities out as they come to me.
After reading your most current post the photoshop idea would not achieve what you want. It would basically have both images as solid only having the background of the top image eliminated so the bottom image would appear in the areas of the top image that are not transparent. Though you could adjust the opacity of the bottom image.
For some reason I am thinking that you want something that, in old time film photography, would be like two exposures on the same frame. The intial exposure being a photo of his grandfather and the subsequent exposure being your son but you want the subsequent exposure to be more prominent.