From all that I've read, it stands to reason that it's best to battle the beast early, when the burden is low and before it mutates.
Unfortunately, cancer is not reasonable. Some therapies don't do a thing until the cancer has mutated into a form that is susceptible to that therapy. For example, we just learned that docetaxel does nothing until after significant metastatic progression has occurred. Provenge also did not seem to do much for non-metastatic, hormone sensitive men. The timing and sequencing can make the difference between a therapy working or being a waste of money.
Mutation has already occurred, or his PSA would be undetectable under androgen deprivation,and would not be rising.
If you want to pay for something experimental, you might consider local treatment of the prostate (RP or RT). A recent clinical trial showed more benefit among men whose cancer is less progressed. If money is not an impediment, there are several therapies that are more promising than Provenge that would be available off label.