I like your husband's name.
I'm sorry to hear that his HIFU failed. The first task is to try to figure out where the failure occurred: local (in the prostate), regional (in the area around the prostate), or distant. If it's still in the prostate, the prostate bed, or the local lymph nodes, he may be a candidate for salvage radiation therapy. This is specialized treatment, and he may be better off being treated at a major treatment facility like the Moffitt Center in Tampa or perhaps the Dattoli Cancer Center in Sarasota.
I don't know why his bone scan is equivocal, but the metastasis often has to be large before it can be picked up in an ordinary bone scan, which is a type of PET scan. There are better PET scans available now that can pick up smaller tumors. The Moffitt Cancer Center may be able to give him a NaF18 PET scan, or they may know where you can get one locally. Even better would be a C11 Acetate PET scan, which is available at a few places in the US, or a C11 Choline PET scan, which is only available at the Mayo Clinic in MN, as far as I know. It may well be worth the one day trip to one of these places to get such a scan.
There is an advantage to beginning treatment sooner rather than later, so it is important to get the scan done quickly. But he will have to have it before beginning hormone therapy, which would render the tumor invisible.
"Active Surveillance" is a term that is only used before any treatment is given, so he is not on Active Surveillance.