Quite common to have a few cancer cells left in the prostate bed (the place where the prostate used to be), and the slow rise in PSA suggests that it may well be that. If it is, and scans show nothing more amiss, a course of radiotherapy is the treatment most likely to be offered. That would still be given with curative intent.
Next stage will probably be scans similar to the ones he had during initial diagnosis.
Try not to worry. This is a not uncommon situation.
Best wishes,
Chris