Hello and welcome to the site that no one wants to join.
Sorry to hear about your father in law, I know it's a very worrying time and I'm wondering if your FIL has been referred to an oncologist since they are often a more appropriate specialist than a urologist (though some people here are very happy with their urologist treating them). It would help a little if you were able to tell us his G score, and current PSA although with many mets and such a high PSA he must be very high risk.
Has he had any contact with the local MacMillan or hospice staff? I tend to recommend an early referral as they can talk to him about his Cancer and are brilliant at advising on pain relief and can coordinate between GP and hospital on his behalf.
I don't think anyone here can say if the pain you mention is the Cancer spreading further, I think this is something he should discuss with his GP, urologist or his specialist nurse. My own husband had some dreadful pain and awful blood results last year, we all thought (including the oncology team) that it was the Cancer and it turned out to be a blocked bile duct.
It's such a worrying time, but if it helps at all, we have guys here with even higher PSAs and they are still going strong. Depending on his current general state of health and his age there are other courses of treatment that can be offered. I mention age purely from the point of view that my own FIL didn't want any treatment, at 90 years old he wasn't interested.
Please let us know a few more details if you can and definitely get a referral to MacMillan as soon as possible.
PS, I'm not sure if you know but you can click on a user name and see the profile information of other members, it often helps to get an idea of how others have fared.
Kindest regards
Allison