I believe you mean the perineum; the prostate is inside the body and I wouldn't recommend trying to insert the standard Hitachi head there. Just be sure you don't pinch his sack in the process when you're down there; it might be more pain than pleasure.
Some men can experience "external" prostate stimulation, if you apply pressure on the perineum. I'm not sure if it would be as pleasurable as a "direct hit" on the internal portion of the p-spot, but it might feel nice. Does your partner have any interest in anal play? I think the Hitachi wand has a few attachments that might be shaped to provide direct stimulation to the p-spot, but it might be a little too intense for guys who aren't that interested, especially when you flip on the vibe.
My wife and I tried internal prostate play many times with little positive results but when she applied external pressure to the perineum it was very pleasurable ,especially if combined with a blow job