What Causes Persistent Fever In A Child Suffering With ALL?

My son is seven years old. He was diagnosed with ALL at 17 months and spent 3 1/2 years on chemo. His last treatment was in Dec. of 2012. Three weeks ago he was diagnosed with strep throat. He completed his antibiotic, but was still running a fever. They checked and the strep was gone so they assumed it was a virus. One week later, we had CBC panel done on him and there is no sign of relapse. Long story short, It has been three weeks and he has a fever one day and he is fine the next. He is lethargic and not eating well. CBC was fine except for elevated WBC (which they said happens with any illness) and cultures for bacterial infections were fine. I am desperate at this point. He is my son. I know him better than anyone else. Nobody seems to understand that he is just not himself. Where do I go from here? (made 2 trips to the ER, 1 to pediatrician, and oncologist doesn t want to see him as long as he has a fever so that he doesn t make any children in treatment ill).