We don’t know for sure that he didn’t have a secondary bacterial infection in his lungs. I’ve contracted pneumonia more than once as a result of the flu, and each time it responded well to antibiotics, meaning it was bacterial and presumably a secondary effect of my lungs initially being compromised by the flu virus.
You should know whether the pneumonia is viral or bacterial for sure BEFORE you take antibiotics. Antibiotics are a double-edged sword, and taking them while fighting a severe viral infection like COVID-19 could be very damaging.