Lexi26, my case doesn't quite relate to your husband's, I had surgery without adjuvant RT but then after a rise in PSA had HT and salvage RT six years later. However what might be relevant was the advice of the oncologist when my PSA rose above 0.2 that he saw no point in me having a scan. He said that in his experience scans couldn't be relied on to provide information when PSA was less than 0.5 and that it would be best to start the salvage treatment at a lower PSA.
(Basically, with PSA at 0.3 or 0.4 he wouldn't take a negative scan as meaningful, there would be too much risk in not going ahead with salvage treatment, so there was no point in a scan which didn't guide the clinical approach).