When I first tried to stimulate my b/f prostate, neither one of us knew exactly where it was. An older male friend gave me this tip and it worked like a charm:
Have him in a sitting position (couch or chair works best) leaning back, get between his legs, and put your fist under his balls (knuckles up, so that his balls lay on the back of your hand). Use your mouth and/or other hand on his penis, and as he gets more excited he can lean forward on your fist, until he finds the spot and pressure that feels best.
If you guys want to find it by your self, use a ball in place of the fist, but be sure to be on a padded surface, so that the ball will sink a little with the pressure.
Happy Hunting