Salvage radiation is a possibility, and he may be lucky, and experience few symptoms from it.
Another option is to do nothing. He has no symptoms, and it might stay that way a long time. If he does eventually have some symptoms, they can be treated with hormone therapy then. There is no survival advantage in going on hormone therapy sooner rather than later, so why rush into it?
"Our study suggests little or no survival benefit of immediate ADT initiation for immediate ADT initiation compared with deferred ADT initiation (at clinical progression or at least two years after PSA relapse) among prostate cancer patients with PSA-only relapse."Immediate versus deferred initiation of androgen deprivation therapy in prostate cancer patients with PSA-only relapse.