It's very difficult isn't it when our husbands go all stoical on us and try to save us any worry by not discussing their cancer.
I had hoped that a member with the experience or knowledge would have answered you as I can't say what is usual, not having been in your situation.
I do know however, that PC doesn't have a behavioural template and just because one person has a low PSA and was terminal it doesn't mean that it's normal that all low PSA is aggressive and life threatening.
We are all different in how our bodies (and indeed how our minds) deal with cancer. I would just accept that your husband is having treatment that is successfully lowering his PSA. The fact that he is on a drugs trial means he is carefully monitored too.
I hope somebody else comes along to reassure you.