Dad is 59 and had knee surgery 4 weeks ago. he sleeps all day long and won't eat anything and says his whole body is achy. what could be wrong?

Tired after TKA: Consider postoperative anemia. Check hemoglobin, hematocrit, possibly iron or thyroid levels as well.

Medication: His response is not normal. I would be worried that his issues might be related to possible pain medication or even blood thinner use. I would suggest checking with his primary care doctor, especially if he might have any other medical conditions (hypertension, heart disease, diabetes, etc).