I agree with Nomar. There is no point to having a PET scan if his PSA is so low that it wouldn't show anything, even if it's there. I can certainly see why you'd want one to help you decide between salvage radiation or permanent hormone therapy. Sadly, at very low or stable PSAs, none of even the newest PET scans are very good at finding distant mets.
Maybe the following nomogram can help you decide whether it's worth having salvage radiation. But it requires a PSA after surgery to fill it out.
/www.mskcc.org/nomograms/prostate/salvage-radiation-therapy