A few questions:
> how old is he?
> what was his Gleason score?
> where did his pre-treatment PSMA PET scan show the cancer was located? And what were those SUVmax scores?
> what was his testosterone level at each pre-treatment and post-treatment PSA tests?

I’d wait for a 3-month post-radiation PSA test before jumping to any conclusions.

However, with no significant testosterone being produced and if no metastasis initially, PSA should be undetectable.

> since he hasn’t been on any ADT, only Abiraterone (Zytiga) which is an androgen receptor pathway inhibitor (ARPI), any residual testosterone shouldn’t be picked up by any (potentially) remaining prostate cancer cells.

My experience with having 28 radiation treatments with ADT (which has the same temporary effect as an orchiectomy; my testosterone level went as low as 3.0): on the last day of radiation, my PSA was 0.224; at 6 weeks post-radiation my PSA was 0.008 (and remained there for 6 months). Now, almost 5 years post treatment, my most recent PSA (in December) was 0.314.

If his testosterone levels are single-digit, but PSA is still elevated, then yes there is need of another PSMA PET scan.

(Also - and this is just a curiosity question - why did he choose an orchiectomy?)