It is strange that it is this bad at such a young age. One thing which needs clarifying is the words 'terminal' and 'incurable'. People can have incurable prostate cancer for decades, no existing treatment will get rid of it, but death is not imminent. Terminal is usually used by a medic when they think the patient will not last more than six months. The lay person may hear a medic describe a cancer as incurable and think that means it is terminal, so make sure you are clear as to whether he is incurable or terminal.
Hormone therapy (HT) is usually the starting point for incurable or terminal cancer. The old protocol was to keep using stronger drugs as the original ones failed, chemotherapy was quite a long way down the list, it usually took about five years before chemo was appropriate. A newer protocol is to try all(well a lot) of the drugs immediately and knock the cancer back for a long time (a few years).
His progression to very bad symptoms is either a reaction to the drugs or rapidly progressing cancer. HT can have unpleasant side effects but not as bad as you suggest. It can cause bone fractures, but usually it is after several years.
His PSA levels usually suggest how severe the cancer is. If these are in the hundreds it does imply a very aggressive terminal cancer. He may respond well to the chemo, but sadly you are probably best preparing for the worst.
I hope things turn out better than they currently look.