Focus on calories, not nutrition, for a cancer patient
You want to serve healthy, balanced meals. But what matters most is that he keep his strength up. If all he wants to eat is sweets, this isn't the time to worry about that. In fact, weight loss is the enemy of cancer patients, so high-calorie foods are preferable to low-calories ones if he's struggling to keep weight on.
There are also some easy ways to boost the calorie content of the foods he's already eating. Substitute whole milk or cream in hot chocolate, oatmeal, and other cereals and any recipe calling for milk. You can add whole or whipping cream to desserts, pancakes, and waffles and even to mashed potatoes and pureed vegetable soups. Use butter in sauces on vegetables and fish, or make a rich gravy for meats. And top cakes, pies, and other desserts with whipped cream, ice cream, or cream cheese frosting whenever possible.
If there's a dish that spurs his appetite, don't worry about whether it's what he "should" be eating. This isn't the time to be overly concerned about fat, cholesterol, or other nutritional issues. Think about easy-to-eat snack foods that he has liked in the past, even during his childhood. Muffins, pudding, ice cream, cheese and crackers, and peanut butter are often popular with cancer patients. One caveat, though: During periods of extreme nausea and vomiting, when nothing seems to tempt him, you might want to avoid serving his favorites. Otherwise something he really likes may become associated in his mind with nausea, removing another key dish from the list.