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This condition of unexplained fever is called PUO, or Pyrexia of Unknown Origin.
While he is under investigation, you should rehydrate him adequately by having him drink large amounts of electrolyte fluids, electrolyte solutions can be bought OTC at pharmacies, you should discuss this with the pharmacist.
PUO could be both due to infectious diseases and non-infectious causes. There are a large number of infectious causes such as Miliary Tuberculosis, Lyme disease, Mycoplasma, Riccketsia, etc. Non-infectious causes include hyperthyroidism, cancers, etc.
The person to see for this is an Infectious Diseases Specialist for this. You have already done so and he also apparently failed to find any cause. Most of the other causes in the list also like cancer, etc. have been ruled out. In the circumstances, he would be considered a true PUO. Some cases of pyrexia of unknown origin really remain 'of unknown origin' despite many investigations. Of course it cannot really be on unknown origin, there is probably some cause which remains undetected either because the disease was subclinical or we not have the tests yet to detect the disease. Most such cases get better by themselves without the cause ever being found out, but in some cases the subclinical disease eventually becomes clinical, for example, a very mild hepatitis undetected by tests later becomes detectable.
Since you are following the correct protocol, you should continue to persist with this. Work closely with the infectious disease specialist and the other doctors. You should look up these sites for more info:http://www.patient.co.uk/showdoc/40000333/ http://www.emedicine.com/med/topic785.htm I hope this answer was helpful. If you need more information, feel free to ask. For accepting this answer, please click on the green 'accept' button below. Feedback/Bonus will be warmly appreciated.