so...im actually going to post a serious response, because i've been thinking of making a zine on this (and prostate health) anyway...
rubbing or pressing the taint during or before orgasm wont stimulate the prostate. however, pressing the taint before and during orgasm DOES stop a man from cumming (while still experiencing the orgasm, he wont ejaculate)... but thats another subject. i personally have a cheap prostate stimulator butt plug for this purpose, since its difficult to do a reach around in most positions, and any amount of fingernails can hurt if you aren't doing it to yourself. i have a water-based lube (personal choice since it doesn't contain animal glycerine). i bought a small toy when i got mine since it had bn a while since i had done any anal play, but now i kind of wish i had gotten a larger one. maybe somewhere in the middle...ask your man.
my toy has a small loop on the end for your finger to go through, which makes it easy for me to use when my girlfriend is on top or fotr her to use when i am on top... but its a matter of personal preference. as for the shape, its nice to have something with a bulbous end, and a curve...the male "p-spot" is in roughly the same place as its female counterpart.
as for prostate massage not during sex...he should do it himself first, a good position in on the knees, head down. then use the middle finger (obviously lube it up) and basically go in a couple inches, while applying pressure towards the stomach, until he finds a walnut size lump (i had a hard time finding a lump myself, but i definitely felt when i hit the spot) and slowly apply a "come here" motion while applying pressure, starting behind the prostate, and extending down past it. just keep doing that and eventually he should feel like he's going to pee...he wont, but sooner or later a small amount of discharge should come out of his dick. its asically just the non-sperm part of semen, and its actually build up, and full of toxins that he just expelled from his body, so wipe that shit up and throw it out. thats a pretty general idea ut i think its pretty good...let me know if you want more info