(To Bloomer:
As far as I know, it feels soft (yes, like a little lump) and spongelike.)
Cardinal explained it perfectly. There's nothing to add.
But please be aware that for some men the mere thought of any kind of anal penetration is completely unpleasant (not to say scaring). I think, it has to do with a kind of homophobia or painful/embarrassing memories of prostate exams.
Before you try it, you should carefully make sure that your boyfriend at least COULD like it (unless you already know). Perhaps by unobtrusively stroking his butt, while you kiss his neck, slowly getting closer to the "crack", and so on.
Even with lube any penetration is painful as long as the sphincter is not relaxed (of course for both men and women). First you have to make him get used to it, by softly stimulating it with little circular motions with one (lubed) fingertip or even better: with the lower joint of your thumb when you make a fist. That's a perfect little knob to make the feeling more diffuse. Take time, before you (slowly!) insert anything. Again it helps, to kiss his neck simultaniously (Makes it feel less "clinical".)
Another aspect: When the whole g-spot thing became en vogue twenty years ago (in fact, it was already discovered in 1950) every woman felt the pressure to have "million dollar orgasms". Those who didn't manage, felt insufficient, like some women (sadly) still do for not having orgasms only by penetration. And every man felt the same pressure to FIND the g-spot.
We all know sex is better without pressure.
As Cardinal rightly said, not all men are able to achieve prostate orgasms (I guess only few, and some don't even want it to be touched.)
For me the "pressure on the perineum" thing still works best to improve the feeling of any penis stimulation (by far).
(As you might have realized, English is not my native language, but nevertheless I know what I'm talking about).