If you want to help your prostate in every way you can then I would recommend taking a good quality supplement that specifically targets the male urinary system called saw palmetto.
This has been known to help men suffering from prostatitis significantly.
Zinc and Selenium are also essential supplements for male health in regards to his reproductive and urinary systems. If you can take them as part of a high-quality multivitamin that would be preferable as the synergy that the body gets from taking multiple minerals etc together far outweighs just taking one vitamin or mineral by itself.
Eg: taking vitamin c without zinc is pointless, you need it to absorb vitamin c properly. (You'll note that virtually all supermarket brands use the the cheap vitamin c in it's acidic form and without zinc)
I would recommend supplementing your diet with high-quality anti-oxidants like grapeseed extract. This will boost your immune system to help fight the problem.