dear user we know that recurrent fever or prolonged fever or fever of unknown cause resembles worry to every person but we shouldnot get worry and try to find the cause.......your child was having sinus infection then he took two types of antibiotics that may lead to bacterial resistance of antibiotics as administration of different types of antibiotics causes the bacteria to accomodate and get resistance...........may be your child have another bacterial infection rather than sinusitis and not treated.........simply you should do urine analysis test for him to investigate if there is urinary tract infection or not ....also you should do widal test to indicate if there is typhoid disease that causes recurrent fever... another test to do is blood culture test ....low grade fever reappears means that there is bacterial infection that is not diagnosed untill now ....doing these three simple tests will help alot and try to see your doctor infectious disease specialist or peditrician...........also you should notice your child if he has any troubles rather than fever such as changing urine color or diarrhea or constipation or any rash appears on him and please send to us about that...we can help reach diagnosis ........and any abnormal symptom you notice will lead us to the correct way of diagnosis and treatment..............i wish your child happy healthy life