Lupron can make the scans show up as false negative? Does PSA still have to be high in order for the scans to show mets? At this point, we have no idea what his PSA level is.
Yes, Lupron will almost certainly cause false negatives. That's what Lupron does - it shrinks prostate cancer. Depending on the scan, PSA has to be greater than 2 or rapidly rising for a met to show up on the best PET scans (and FACBC may be among the best.). His PSA is almost certainly well below that level because he has been on it since January and there is no reason to believe he would be castrate-resistant at this early stage.
Most people in Randy's situation do exactly what you did - get a bone scan pre-Lupron to check for larger occult metastases, and proceed to salvage radiation. While it's true that a pre-Lupron PET scan might or might not have found smaller occult mets, thus ruling out salvage radiation, you have to go with the info you have - you can only proceed onward at this point with the salvage radiation.