Sorry you find yourself here.
You haven't given much background, so it's difficult to comment.
It would be useful to know his PSA at diagnosis and all subsequent readings with dates, his staging (TxNxMx), gleason score, and what was learned from the histology of the removed prostate.
Has he had a PSMA PET scan to locate the remaining prostate cells?
Or maybe they know from the histology?
If you can fill in as much detail you know in your profile, that's useful for answering questions.