Skip the gym entirely - he will need ALL of his energy for healing. He can always go back once his immune sysem has recovered and he will need too actually. Cancer fatigue comes with the the disease and is a well known side effect so he will have to find a state of acceptance. I know from personal experience how difficult that is but he is fighting for his life. Late in Tx I was so wiped out I had to plan my day around when I had the most energy to sit in the shower or even drink water or take in food. If he can take short walks that should be good enough during treatment.