Hello I understand your concern. Persistent fever can be due to any infection in body. It needs to be evaluated further based upon other clinical symptoms and if needed from certain investigations. I advise you get him first checked by a good physician to obtain a suitable medical history and physical examination. Does he have cough,cold/burning micturation/abdominal pain,diarrhea/abscess anywhere over body ? Please get his investigations done like blood haemogram,liver function test,X ray Chest,Urinanalysis.This will help in ruling out urinary tract infection,gastrointestinal infection or lower respiratory tract infection which are more common in diabetics. Further management will depend upon empirical antibiotic therapy.If there is non response,specific swab cultures and drug sensitivity needs to be done depending on which body system is involved. Also get his fasting glucose to see if his diabetes is under control.It may be adviced to shift from oral medication to insulin injection till this acute illness lasts. maintain adequate hydration and healthy diet for general well being. Hope this information helps. Wish him speedy recovery.