Talk to General Physician on Low Grade Fever And High Pulse Rate

My 14 year son have low grade fever at evening 99 to 100 from 21 days.he have taken cefaspan ds and azomax 7days but temperature is not goung to settlte along with his heart beat is also increase when he stands it goes to 180 .his echo is normal.doctor recomend him inderal 10mg daily.thrice a time.i am very upset his temperture 2 weeks before his blood culture was normal,cbc normal esr normal.please tell me what the cause of fever?