Welcome to HCM,
After going through history of your brother,fever
and X ray
.He has not respoded to antibiotics and his problems did not resolved.
It is possible that the disease has progressed and cause may be tuberculosis
Even infiltration and cavity in X ray suggest Tuberculosis.
If there is cough and he is bringing out sputum a test of sputum to see AFB is confirmatory,but if no sputum is there sample may be collected through bronchoscope.
If chest physician suggested you have faith in your Physician.
He should improve in few weeks time but treatment for tuberculosis is to be taken for long time may be 6 to 8 months.
Even in this forum you can ask a chest Physician or Pulmonologist and you can send pictures along with your query to get opinion.