Cancer patient having fever. What can I do to help him?

Call Oncologist. It is important to contact his oncologist about a fever. His immune system may be suppressed and any infection needs to be dealt with promptly.
Fever. If you currently are on chemo and you have fever- -especially if you have a significant low white cells and have a temperature of 100.4 or more, you will need to call your oncologist immediately or er. Febrile neutropenia is a serious condition. If you are not currently on chemo, and if your white cell is fine and fever is only low grade-you may take tylenol/nsaids-and pcp if not getting better.
Tylenol/motrin/PCP. This is complicated, but the short answer is I would use tylenol (acetaminophen) or Motrin and take him to the doctor. I would be more concerned if they were currently undergoing chemotherapy.