hello informations provided by you are a bit less..his age and weight is also required..but still let me try to give a suitable advise regarding it medically this is called as fever of unknown origin..and it has many causes The following conditions are sources of FUO: • Abscesses • Tuberculosis • Urinary tract infections • Endocarditis • Hepatobiliary infections • Osteomyelitis • Rickettsia • Chlamydia • Systemic bacterial illnesses what ever the reason may be..the main underlying cause is some kind of infection and a low immunity statues so my advise is get a few more tests done.. that is for tuber culosis... esr...and other inflammatory marker tests mean while i will suggest you some remedies for enhancing immunity 1) include lot of citrus fruits and antioxidants in his diet.. 2)consume almonds walnut and figs regularly 3)one herbal formula for strength is..boil milk with figs and dates in it for a few minutes and have it two times a day 4)an indian ayurvedic herb called as amla (emblica officinalis) is a wonder herb for enhancing immunity and eradicating infection from the body 5) turmeric also eradicates infection from the body 6) morning walk for 20 minutes is a must try these remedies and get a few more tests done so that the picture becomes more clear best luck