Sorry you're going through this.
In reality, his oncologist probably can't predict what will happen, and plenty of times when they do, it's wrong.
The chemo will hopefully knock the cancer back for a while. They often can't tell for the first 4 or so sessions if it's going to work, because the chemo itself can push up the PSA temporarily. I'm guessing he's having Docetaxal. If that works for a while, he can have it again. If it doesn't work, there's a second chemo drug Cabazitaxel which they might try. Obviously, further chemo depends on being well enough to handle it.
If his mets are only in bone, then there's Radium 223.
If his mets are not only in bone, there's Lutetium 177, but I'm not sure where that is with NICE/NHS approval. It was refused last year but was being appealed. There have been some patients on trials with it which might be another possible route.
If he would be interested in trials, make sure his oncologist knows that.
I would also emphasise you need to get provision for palliative care in place, before it's needed. Talk to the CNSs about it, and see what sort of services such as hospice at home are available and get them engaged, so they're ready to go when you need them. They are often better at pain management than hospital services, and they might make some good suggestions about handling his current pain.