Hi. Would discuss with oncologist to see if he could take some time off from the Keytruda until he gets his strength back, if he finds this new fatigue troubling. My thoughts anyway. Keytruda has a long half life, something like 2 months or more before it is basically out of one's system. Also would check the Keytruda website for side-effects and see what they recommend. The oncologist may have a better answer for him however. Best of luck!