Interesting. While on ADT, it’s important to know testosterone levels.
ADT results in suppressed testosterone, which then starves/weakens prostate cancer cells, resulting in lower PSA.
If ADT no longer results in lower PSA, you’d want to know whether (1) the ADT was not suppressing testosterone levels (thus PSA was not dropping) or (2) testosterone was being suppressed but prostate cancer cells were not responding.
The solution to PSA not dropping low enough would be different in those two cases.

(From the time I decided on radiation treatments and ADT in early 2021, we’ve tested Testosterone level with ADT every time - that’s been 16 tests to date.)

You should ask his doctor why no testosterone tests? He might have a reasonable explanation.