Good suggestions from the doctor above. It's a near miracle that your Dad is cancer free from esophageal cancer. I thought that it had a survival rate of about 5%. Maybe I'm wrong. You can let me know. But congratulations!
Hello and hope you are doing well.
Understand your predicament. Due to inadequate nutrition he could have anemia or vitamin deficiencies which could be causing the weakness or malaise. His symptoms could also be due to depression, due to his prolonged illness. You can talk to his primary care physician, who may arrange for professional help if needed.
Hope this helped and do keep us posted.