Sarah
Although at a much younger age I also went from very fit to almost zero activity. It took nearly 12 months to get over the worse. Thus your dad might start to improve soon. The following migjt help -
1) if side effects are causing poor sleep enquire what help is available.
2) Some guys have found changing from 12 weekly to 4 weekly injections helped
3) Moving from injections to tablets such as Casodex helped some.
4) Bearing in mind his age/quality of life a holiday from HT might be worth enquiring about but that may well reduce survival time, something consultants might not go along with.
Ray