Expressing semen by massaging the prostate is the way a urologist will express semen onto a specimen slide to perform lab tests. I know, because I've had it done. He had me lower my pants, bend over the exam table, and he put a gloved finger up inside my hole and pressed against the prostate while he reached around the front with the other hand and caught the specimen on the slide.
It was a bit uncomfortable and pleasurable at the same time, but I didn't have to tip him, and my insurance paid for it. LOL.
A wand with a mild shock at the end is used on male animals to achieve the same thing. They put the wand up the male animal's ass, then deliver a very mild shock to the prostate and it makes him cum instantly. Not sure if such a device is available or even safe with humans.