Dealing with nausea is one of the daily challenges of caring for someone with cancer, and you just have to keep changing tactics, trying to find things that help even
a little bit. If you've been prescribed antiemetic drugs -- medications that fight nausea by altering the chemistry of the brain -- you can ask if there are other antiemetics you haven't tried. There are many different ones, all formulated to act on different brain chemicals, and some people respond better to some than others.
Also pay attention to timing. You might want to suggest that your dad take his antiemetic medication as soon as he gets up in the morning, before the nausea sets in. When my own father was sick, he'd just throw up the antiemetic, so we had to give it to him while he was still feeling okay. Some medications work to control symptoms once you have them, others work preventively, and that's how antiemetics work. You want him to take them while he feels fine, because as soon as he's nauseous, it sets up a self-perpetuating cycle. Most antinausea drugs take 20 to 30 minutes to start working, so have him wait that long before he attempts to eat. Also, make absolutely sure that your father always takes an antiemetic drug before he has his chemo treatment so it protects him both during and right after treatment.
In some ways, having cancer is similar to being pregnant -- people are incredibly sensitive to odors, and the slightest change in smell can set off the nausea. That means every change in the environment is a challenge. Think of every new smell as an assault and keep your father's environment as status quo as possible, taking whatever steps necessary to avoid unfamiliar or unpleasant odors. For example, the smell of the parking garage at the hospital used to make my dad sick, so I'd have him stay in the waiting room while I pulled the car around.
At home, have your father stay away from the kitchen anytime someone is cooking, even if food is just being heated up or microwaved. Garbage cans might have to be moved outside, or you might have to buy one with an airtight lid. Detergents and cleaning solutions can be huge triggers, so if someone is cleaning, have your dad sit in another room or even outside on the deck in the fresh air. Pets can really be a problem, and that's a hard one. If the smell of the dog is making your dad sick, the dog may have to stay with family or friends. Use a fan to keep fresh air circulating.
If you're going to friends' homes, ask them to put the dog outside, not to use air freshener -- the fewer odors the better. To that end, minimize changes in the environment as much as possible. Let your dad stay at home while someone else runs errands, because even just driving by a Mexican restaurant or smelling the scent of the floor cleaner at the supermarket could make him feel sick.
Again, as with a pregnant woman, have your dad eat small snacks, like dry crackers, as often as possible. He needs to stay well hydrated, because lack of hydration is a problem with nausea. Water is like lubrication for the body, keeping metabolism running smoothly and the chemical balance in tune. Have your dad sip liquids through a covered glass with a straw. People often recommend teas like ginger tea, but hot water can also trigger nausea, so let the tea cool until it's at room temperature when it gives off less odor.
After meals, have your dad sit quietly and not move around much. Try having him sit at a 35- to 40-degree angle and see if that helps. It's a constant process of trial and error; something may work one day and not the next. Just keep trying and don't be afraid to experiment.