Hi, Suzanne, and welcome. A combination of the cancer, the meds and the treatment can interfere with appetite. To be sure your dad has enough nourishment, it may be better to go to snacks every two hours or so instead of three squares a day. This will also tend to minimize the nausea.
As for the meds, your dad needs to understand that this is helping him and he needs to take them. For instance, pain meds are not addictive as long as they are covering the pain. It is when you take more than you need for the pain that they can become addictive.
Your dad is of an age where he would tend not to ask questions. Also, he is male, and we don't ask a lot of questions. It is important that someone go with him to doctor visits and they ask the questions.
Hang in there. Don