My 4 year old is in the hospital and the doctors don't know what is wrong.?

So my son has run fever for 10 days now. It varies from 99 to 104.5 . We were admitted on Tuesday and they have done all sorts of tests and can't figure out what is going on. Up until yesterday, fever was the only symptom that he had. But yesterday he started sounding very congested in the nasal area. Any suggestions of what could be wrong would be great. We've been tested for strep, flu, mono, pnemonia, urine, etc, etc, etc!