You say your step-dad had a by-pass. You also say that you have read about the benefits of certain diets. Did you make any comparison between your step-dad's regular diet and those that you read? You also say that he is on a low-fat diet but is constantly hungry and has mood-swings. Doesn't sound from this distance that his diet is serving the purpose but then we all are our own executioners.

Cholesterol is an essential bodily nutrient that is produced from saturated fats (not veg fats) which just also happens to be highly nutritious.

Grains, seeds, nuts (possibly with the exception of those sprouted) and their products are harmful and tend to ramp-up the blood sugar-level and cause weight gain.

You may wish to read what Dr Mary Enig and Dr Raymond Peat have to say on the subject (try a search engine). An interesting site would be that of 'Weston A Price' for general information on diets.