My father is 82. He has acute pneumonia. Antibiotics does not work on him. What to do?

Maybe resistant. In patients with pneumonia antibiotics "work" if the pneumonia is caused by a germ that can be killed by the antibiotic. If the antibiotic is not working it can be because the germ is not susceptible to the antibiotics being used or sometimes because it is caused by a virus. Most viruses can not be treated with antibiotics.
Recheck. If 82, he will have limited physiological reserve. If he is not improving, or is worsening, then he may need IV abx, breathing treatments, etc. He should be seen in the er.
In hospital. Hopefully he has been hospitalized. If so, then the doctors caring for him can respond to your question. If not, he definitely should be in hospital and probably in intensive care. Hope he gets better.