What a shame -- eating and mealtime should be a source of sensory and social pleasure for your dad and a break for your mother from daily frustrations of care.
Perhaps your mother is trying to control his nutrition because she believes it is something she can control in an otherwise uncontrollable disease process. Wise family members learn that yes/no battles are rarely won by rational arguments about what's "good for you."
Your dad eats slowly, but he probably does everything more slowly because he is easily distracted. He may also be eating less because he isn't getting much exercise or because he is overwhelmed by the stimulation of a full plate with too many choices.
He may eat more with fewer distractions and a more relaxed environment. If he anticipates a battle or some impossible expectation because of your mother's tense body language, he will resist intuitively.
Try serving one course at a time with fewer items on the table. Allow plenty of time to eat. Make sure your mother sits down with him and slowly eats her food. Be sure the TV, phone and/or radio are all off, but sometimes soft background music helps. Try his old favorite foods and remind him of how your mother prepared it according to his mother's favorite recipe or whatever else reminds him of beloved foods or festive family meals. Use mealtime to reminisce about favorite foods and social occasions around food. If he is really distractible, try leaving healthy snacks out on the counter if he paces around the kitchen. He may grab something on the go. If he is losing weight and never liked vegetables or salads, skip them in favor of higher calorie, more inviting snacks. Overall, the goal is to restore pleasure in eating as a sensory and social experience; it shouldn't be t a forced march through recommended daily food groups.