The best way to introduce it is to start playing around with his ass when you're giving him head. Just running your fingers around his taint and asshole will give you a good idea if he's receptive to it. If he responds positively to your touch, make sure your finger is lubed up with saliva and slip a finger in. Anal play can be very pleasurable if done with plenty of lube and patience. It's a short trip from a finger in his ass to finding that magic button. (His prostate)
If he clenches up while you are touching his nether regions, then he will probably object to a finger, But be patient, some men will be reluctant to having their ass played with until they realize the pleasure they can get from it.
As far as prostate massage goes, I recall first hearing about it's use in long term chastity situations. When a dominant wife keeps her man in chastity and plans to make it long term, she uses milking to expel semen from the man to ensure his prostate health without allowing him to orgasm .From descriptions I've read, it allows for the release of semen without the pleasure of an orgasm leaving the husband still frustrated over his lack of release. I've seen videos on the web of experienced Mistresses milking their chaste husbands while still in their chastity cages. One that really struck a bell with me was a wife who milked him onto a plate and made him lick up his semen.