My 10 year old has ran a high fever for 3 days. What could this mean?

Last night it got up to 104 so i took him to the er and they checked for strep, flu, did a chest xray, did a cbc, and urine sample. Everything came back normal and they sent us home and said to piggyback Tylenol and motrin for the next 48 hours so it wont spike. I am really worried about what could be causing this fever. Some background on my son is he has in the past had febrile seizures and cancer diagnosed when he was 6. He has been off treatment for 2 and a half years now.