.."he pretends he doesn't..."
In his debilitated state (or anyone) he can only make himself do these "tiresome chores" (as I believe he considers them) to the extent of his knowledge about the "chores" or tasks and what THEY CAN DO FOR HIM.
Have you already tried educating him on gaining more understanding as to how he got in this physical condition?
Go to Dr. Lorraine Day's web site. She has a list of informative and useful videos. And perhaps you or your dad could pick one he thinks would be of interest or help him in some way. My suggestion is to think about seeing the one called, "Diseases Don't Just Happen."
Once a person truly realizes how he came down the path to where he is, he can stop doing the things (like eating non nutritious food) that are truly harming him.
But also, he has to gain knowledge from his own experience, or have seen the healing experience on someone else in a similar situation. If he is going to use a nutritional treatment, he must do all of it, and not half of the treatment. We are not taught this manner of tending to our health in grammar school or later. And long held attitudes are hard to modify. Try that or some other video.
Or talk to a patient who has recovered using nutrition. There are many out there. It would be nice if you would post here what response you get.
Her web site: