defibrillator implantation recuperation

My father is 76 years old. He had a defibrillator implanted 4 weeks ago. He has been home for 3 weeks and is getting little advise about recuperation. He finds that he is short of breath and is feeling very low. He has no interest in food, but is eating well when food is presented. He wants to know what activities he can do, safely, and if there is any vitamins or tonics he can take to help lift him up. He is normally very active and had no problems before this problem occured 6 weeks ago. He is a non-smoker and drinks very little alcohol. He is at preent recuperating from treatment for prostate cancer.