You can massage a man's prostate from the outside or the inside of his body. And, yes, I'm talking about his butt here.
Why would you want to do this? The prostate is the gland that produces the bulk of a man's ejaculatory fluid. The testicles produce a small amount of fluid, the semen. The prostate creates the fluid that protects the semen from the acidity of the female’s vagina (in a procreation situation) and gives the semen mobility. Massage the prostate and you inspire a longer, deeper orgasm for your guy. Plus it’s good for his health, as studies show it reduces his risk of developing prostate cancer.
In order to do it, first you’ll need a willing partner. If he does not want to go there, then don’t go there. You also need a quality lube, a latex glove, or a condom to wear over your finger(s). You could also try a curved prostate toy. Here are some tips.
Read: How to Massage the Prostate Using a Sex Toy