I am assuming they added the bicalutimide tablets after the decapeptyl began to fail in 2018? If that is the case, it is amazing that the bical / decapeptyl combo has worked for this long! Many men only find that works for a few months. Is he still on decapeptyl or has the hormone been changed since 2018?
The next step might be that they stop the bicalutimide - sometimes prostate cancer learns how to use that as food so taking it away results in a drop in PSA, this is known as an anti-androgen withdrawal response or AAWR. They may also start talking about docetaxel, a form of chemotherapy. The next step is then usually abiraterone or enzalutimide but likely they will want to get him off the bicalutimide and see what happens before they rush to add another treatment.