My kid problem in his urinary tract system

Patient: HelloI have 3 yeas old son. He had three nights of fever (less than one degree) and problem in taking out urine (sense of burning). I went to Dr today and he asked me to go for lab to find the type of bacteria, but my son strongly resist to piss and do the test. I am so afraid and with the note of a pharmacy specialist I started to give him NITROFURANTOIN syrup. It seems he is so afraid to piss because of previous time burning sense (about 12 hours ago) . Please guide me , I know that I can not take a test after using that antibiotic now.CheersHosein

Symptoms: Fever, being impatient, do not like to eat, back and lower stomach pain, walking very little