Hope: A couple of things, the removal of the prostate was probably an attempt to de-bulk the cancer meaning that the prostate was the place where the most cancer was and removing it might allow the Drs. to control the other areas more easily. The Mayo clinic is known to use that process.
Higher gleason scores can have a tendency to not be able to generate high PSA numbers because the cancer itself is badly diferentiated also there are a few types of prostate cancer that just flat out don't generate PSA such as small cell prostate cancer but that should have been caught on the biopsy pathology.
Make sure he is going to a prostate cancer specialist oncologist .
Good luck
David