It sounds like your husband has undergone multiple types of treatment for his diagnosis of prostate cancer.
In addition, to your husband's diagnosis of cancer, and current treatment, other factors
could affect his PSA level such as hormone therapy, and age.
The PSA level is just one piece of information physicians use to determine treatment and prognosis for a prostate cancer diagnosis.
It is typical that an oncologist will look at more of a trend with PSA numbers rather than focusing on one value. A physician would also take into consideration your husband's clinical/physical condition.
To respond to your question about PSA level being undetectable, a normal PSA level for a man without prostate cancer is around 4, given that, a PSA level is typically detectable.
Based on your description of your husband's treatment, it appears that his physicians are following the standards of care related to his prostate cancer.
I would encourage you and your husband to speak with his personal physician related to your questions about being cancer free as well as asking your physician to talk more specifically about his PSA range.