If his psa was rising while he's on Zoladex there'd be some reasons for concern but rising testosterone isn't saying anything direct about cancer but more perhaps about the Zoladex becoming less effective, particularly if your Drs are saying there's no evidence of the disease being active.
Your husband is now nearly a year and a half into his post RT Zoladex treatment and his psa remains low. In theory he's getting close to the 18 month mark at which time many of us go off our HT of one sort or another and see what happens.
Some doctors seem to be overly cautious on this one, thinking perhaps that while someone is either on HT or has low or non-existent testosterone, the chances of cancer cells multiplying are stalled. True, but at some point, whether it be 6 months, 12 months, 18 months .... 3 years, the HT stops. After that there's always a chance that the disease will reappear.
I don't see a reason for adding bicalutamide to the mix now and while I know it can be difficult, you really deserve an explanation for their thinking before taking it on. Given what you've said I'd guess that the answer is something along the lines of the "precautionary principle" but QoL is vitally important too, even if it's often ignored.