Have a full-figured man lean forward from the waist, which puts the chest farther forward than the belt buckle. Have him push his chest out, and pull his shoulders slightly back. Have him lift up a bit with his chin, and push the chin forward. This is about a 100 year-old, professional photographer's BASIC posing strategy for a man with a thick middle.
At appropriate distances with the right lens focal length (meaning NOT from say, five feet away with a 35mm lens!!!), whatever is closer looks ever-so-slightly bigger. It does not matter if the man is seated, or standing; his body needs to be posed, properly, to de-emphasize the thick middle and the under-chin areas. Proper body posture for photography is often NOT the way people will normally stand, unless they happen to be professional models, entertainers, or TV personalities or performers who have been taught how to position themselves to look their most flattering.
The thing about the shot of your father-in-law is that his necktie "slopes" down his chest and upper belly, on a very visible diagonal, which just draws attention to his girth. His under-chin area overlaps the collar on the white shirt, and even has its own shadow. These two things are really serious flaws in this as a classic executive portrait. The shot is very,very un-flattering because of the way he was posed, and the way the camera is positioned in relation to him.