This is a tough one, both for your doctor, and your family. It must be very difficult not knowing what is going on. It's hard for me to make suggestions, because I don't know what testing has already been done, and obviously I can't examine your son. It sounds like your pediatrician is trying hard to pinpoint this. Here is some of what my research on the subject found:
This is an interesting article about the possible rheumatological causes of recurrent fever in children (up to 20% of pediatric recurrent fever is due to rheumatologic causes):
http://www.pedrheumonlinejournal.org/october/fever.htm
Taken from the textbook "In a Page- Pediatrics Signs and Symptoms", the differential diagnosis (ie- the possibilities) for a recurrent fever is:
*Repeated viral infections
–Most common cause of recurrent febrile episodes in childhood
–Start of day care or change of geographic location may be related
*Urinary tract infection (UTI) -May be self-limited but recur especially if underlying anomaly exists
*Epstein-Barr virus (EBV) -May present with recurrent febrile episodes due to one initial infection
*Other specific viral syndromes
–Parvovirus B19
–CMV
*Immunodeficiency
–Repeated bacterial infections should lead to investigation of immune status
*Dental abscess (non-dental abscesses typically present with prolonged daily fever)
*Chronic meningococcemia
*Acute rheumatic fever
*Inflammatory bowel disease (IBD)
*Juvenile rheumatoid arthritis (JRA)
*Behçet disease
*Tumor necrosis factor receptor-associated periodic syndrome (TRAPS) or Hibernian Fever - Autosomal dominant disease with fever, myalgias with migratory pattern, conjunctivitis and rash
*Familial cold autoinflammatory syndrome or familial cold urticaria
–Rash, fever, arthralgia, and conjunctivitis
–Precipitated by exposure to cold
*Muckle-Wells syndrome
–Similar presentation to familial cold urticaria, but symptoms not triggered by cold
*Brucellosis
–Most prevalent around the Mediterranean and Arabic countries, also present in South America and India
*Yersiniosis
*Typhoid fever
*Rat-bite fever
*Malaria
Obviously many of these can be ruled out based on your son's history, and many of them are very very rare (at least in the US). If there are no answers in the next round of blood tests, I might consider seeing a pediatric rheumatologist or pediatric infectious disease specialist, but you can talk that over with your current pediatrician. In one review of 3 studies on recurrent fever of unknown origin, between 43 and 52% of cases remained undiagnosed after testing, so don't be surprised if you have trouble finding an answer.
I hope everything works out. Please keep me updated.
JMK MD
this must be very frustrating. Has he or does he complain of any pain anywhere, ie teeth, headaches, joints,abdominal pains? Has he had any scans of his body done to look for any infections or walled off abscesses? just a few thoughts