Hi, SoSad

You mentioned that "He finished his last radiation treatment last week."

You can give your dad some good news - this is the worst week! Next week may not be so hot, either, but the week after will likely be better. He needs to think in terms of weeks or months, not days. It is VERY frustrating, especially if your dad is used to being active. My husband needed a lot more sleep and rest after therapy - that frustrated him, too. However, the combination of gradually increasing activity, and rest when needed, helped him get back most of his strength and stamina.

As Julieann noted, nutrition and hydration are key for healing and feeling well!
Maria