There are prescription medications, and OTC vitamins to increase the appetite..Marinol, megace, decadron, testosterone, insulin, melatonin, omega 3 fish oil, amino acids, wine, caffeine, but check with the doctor. Adding more fats to the diet will increase appetite, taste, lack of fullness, add more calories. Carbs will fill you sooner that fats will, and who can ever eat one chip or a nut. Adding oil to a drink can be done easily, a little sugar more so in it's effect or misting food with a sprayer may help with both swallowing, but also appetite, taste. Milk, creams, butter have the same effect. He may also have a food, taste aversion from associating the nausea with eating, vomiting from treatment, and may need to be deconditioned.