I tend to be rather optimistic, at the start of my own diagnosis I was positive, and assumed it was probably not cancer; as each test came in the probability of it being cancer went up. At the end of the diagnosis process I was diagnosed with an aggressive cancer (G9 T3 N0 M0) but it hasn't changed my positive outlook.
So looking at your hubby's results, I'm not seeing a massive problem (it could be me being over optimistic). I think your hubby still has a prostate (he didn't have a prostatectomy?, if I have got this wrong don't bother reading anymore of this post, as I am posting under a misapprehension).
If hubby has a prostate and is not currently on HT then any PSA under about 2.1 is not likely to be a problem. If he had active bone mets I find it hard to believe his PSA would still be only 0.1. A bone scan doesn't detect cancer it detects abnormal bone growth, now in a cancer patient with a high PSA that is probably caused by mets, but it could be caused by some other bone disorder (arthritis) https://www.webmd.com/a-to-z-guides/nuclear-bone-scan
So I don't want to give you false hope, but if I were you I would not be assuming the cancer had returned, just that he had aquired some other crippling bone disease instead (last bit about the crippling bone disease is a joke, I have a very dark sense of humour)