I think I would just go along to the appointment and wait to hear what the onco thinks. You could check that dad is at or below castrate level, this indicates whether he is hormone refractory (the cancer has learnt how to survive without testosterone) or whether the HT he is on is not knocking the testosterone back successfully. Next steps usually vary depending on the above.
You could also ask whether there is anything that can be offered for the bone pain - a short zap of RT might be an option but depends on whether he has had RT before, where the bone mets are, etc.
Do you know exactly where the bone mets are sited? If he has spinal mets you might want to talk through with the doctor the symptoms of spinal cord compression .... it is rare but best to know what you are looking for and what to do if dad shows any signs. If they have never discussed with you where the mets are, maybe now is the time to ask for details, especially with new scan results.
I hope it goes well on Wednesday