Here is the complete picture marked up to show Charles Homer Sr. What it is really is in the picture is the man's right hand. If you zoom in on that spot in a high resolution image, the detail of the hand gets better and the father image dissolves.
Look at the bottom right of the painting. There is a redish post. Above the post can be imagined the face of a man with large brown hat. His right eye is looking right and upward toward the location of the image of Charles Sr.