You CAN stimulate a man's prostate by using your finger. There are diagrams readily available on the internet that show where the prostate is. All you need to do is to insert a well lubricated (and hopefully short fingernail) finger in his rectum and rub away. I have heard of people using a technique called "milking" the prostate whereby a particular technique is used to basically give the guy a hand-job while you massage his prostate and presumably it's nirvana for the guy.
I am a guy and unfortunately I don't have any experience with "milking" the prostate, but a while back if the urge got to me I would insert a finger in my bottom while masturbating and I have to admit that the result was awesome. It's particularly awesome when cumming because through the rectum, you can feel the contractions of his orgasm. I imagine that if a woman could get past the grossness of putting her finger in the guys rectum, she'd probably enjoy feeling the pulsations of a good orgasm.
I've also heard of guys going to the doctor for a digital-rectal exam to check the prostate and if the prostate is touched in a particular way, the guy "shoots" ejaculate out of his penis right there.
Now, I think that most things involved in guys rectums, ejaculate, etc is a bit disgusting, but I don't mind being honest with you ladies on this site because it's all for education.
If you have any questions about stuff like this, even though I'm pretty inexperienced in these things, feel free to ask and I'll answer honestly. Also, if you have something I don't know anything about I "may" be able to try it for you to see if it has any validity before you go trying something new on your man.
Cheers.