They use SBRT to zap metastasis outside the prostate. They do it for lymph nodes and bone metastasis. Theoretically, they could do his prostate and also then zap the visible metastasis.

They are now using SBRT to treat the prostate bed, Usually called salvage radiation, They might be able to do that for him, it has many fewer treatments.

It might make sense to get a second opinion from another center of excellence or a Genito Urinary Oncologist. See if there might be another option.