Metastasis can still continue to occur, even while on Eligard. You want your PSA below .1, below 1 could mean a lot of things, what type of numbers have his PSA tests shown?

It is not uncommon for metastasis to the bone to cause pain.

I get lower back pain occasionally, and the only metastasis I have had on my back was on L4 and I’ve had it zapped. I’ve had occasional pain (once or twice a year) in my lower back for 25 years, before I had cancer. When I get the pain I put on this belt I have and within a day or two it goes away. So getting lower back pain is not necessarily tied to the cancer, but it is a possibility that it is caused by the cancer.

Unless you can figure out exactly where the pain is and if it is in the same spot where a metastasis exists, can you figure out what exactly is causing it? The next time he complains about it try and locate it right then so you know where it is actually occurring.

If his PSA is above .2, or a little bit higher, he could get a PSMA pet scan to see if there’s a metastasis in that area of the back. Once you have the scan done you could arrange to have SBRT radiation to that spot on his back, if the scan shows a metastasis.