Hi ! Does he have any burning urination or increased frequency of urine ? Sometimes a silent urinary tract infection can give rise to such fever in this age. I am not sure to which region you belong to. In developing countries , I would suspect either pulmonary or extra pulmonary tuberculosis which is commonly associated with fever, loss of appetite, and loss of weight. Sometimes, nothing comes in the investigations, but on therapeutic trial of anti tubercular drugs, the patient improves proving the diagnosis. If the fever persists, he should also go for an X ray chest and also an ultrasound/CT abdomen. Enteric fever is also a possibility, and a repeat widal test and typhi dot test would be helpful. Last but not the least, in this age group one has to exclude possibility of a focul of malignancy anywhere in the body. I think your family physician have to rule out these possibilities with help of reports of investigations which are due. Wishing him an early recovery.