First thing that is really important is not to panic. My dad had the same situation and his psa was 23. Your father should have a DRE (my dad's was negative). Then some urologist might do a round on antibiotics to see if there is some type of inflammation or infection that is causing the high PSA. Do your research and make sure you chose a highly recommended urologist!!
If his PSA tests high again after antiobiotics or some might suggest a biopsy right away. It can be very scary hearing that high of a psa. I remember looking online and not finding much mentioned with a 20 or so. My father did end up having the prostate cancer, but he had surgery and recovery was quick. His PSA is now undetectable. Just support your dad and try and educate him about the disease. One last tip. Get a folder and keep copies of all paperwork, information, lab results, etc. It is easy to get confused.
Best of luck to your dad.