I am sorry to read that your old man has had biochemical recurrence, as I have two friends who had prostatectomies at the same time as me within the last year, and despite their massive expenditure, costing £££££, with the best British private surgeons and surgery, and they both have suffered the same.
One, had very slight increasing PSA, under 0.1, but an expensive PET-PSMA found nothing to aim at. Nevertheless, they are blasting him with thirty-odd fractions of RT over the whole prostate bed.
Hopefully, salvage RT will knock it on the head in your man’s case.
Best of luck!
Cheers, John.