You can research doublet or triplet therapy for prostate cancer and the results will help you answer your question.

I'm a layman, not a medical professional so keep that in mind.

Basically, there's three ways you try to stop the role of testosterone in prostate cancer:

Production from the testicles.
Production from the adrenal glands.
Uptake by the prostate cancer cells.

The firmagon does the first

The other two get after the 2nd and 3rd ways testosterone is involved.

He may want to talk to his medical team about triplet therapy, adding chemotherapy or radiation if his health permits.

Something to keep in mind is a large drop like that in PSA may indicate that the androgen sensitive cells have largely died off in his early treatment. What's left are some androgen sensitive cells thus the continued use of Firmagon. There are cells who are not sensitive to the Firmagon thus radiation and, or, chemotherapy may kill those.

Anyway, search doublet or triplet therapy and discuss with your medical team.

Here's one link to introduce you to the subject - https://pubmed.ncbi.nlm.nih.gov/36058809/

Kevin