Recurrent fever with high temp 102-103f.

Hi good morning My son is 3 year old . His weight is only 13 kg. He is suffering from recurrent fever for 3-4 days with high temp 102-104 f . And subside by igugesic plus syrup.. And after that cough starts This usually happens in each 2 months . Why is it so. Every time go to doctor. They write n number of medicine but don't provide permanent solution. They even not interested in any blood test. They say he will recover just give medicine. So i have posted here . Why does he get frequent fever . Because of this he is not able to gain weight. Look like lack of body fat . Height is more as his age. Height 100 cm Weight is 13 kg. BMI is 13. Pls suggest test to diagnose infection or any other issue. Then discuss after reports