The first thing to remember is that any kind of unwanted or unexpected anal penetration can be very painful, even traumatic for the recipient. Most people who enjoy and/or desire anal play to be fully aroused before it will actually feel good.
You could get him interested by doing a quick google search of "prostate massage." There are tons of articles by men attesting to the great pleasure they receive from a prostate massage, not to mention the health benefits. You could also try pressing into his prostate from the outside (doesn't work for some men) the next time you're going down on him to see if he likes it. To do this massage the area below his balls in a circular motion. Only proceed to the next step if he's interested.
Some tips for making it a pleasurable experience for him:
-Give him a body massage before hand, so he's nice and relaxed
-Have him bring his legs up towards his chest, this makes it easier to reach his P-spot.
-Be *very* liberal with lube & begin by lightly rubbing the outside of his anus
-Never insert anything up his ass that doesn't go in easily. He'll loosen up when he's ready for insertion.
-Enter slowly, & use very gentle strokes at first (use the "come here" motion upwards towards his penis with your finger to locate the prostate)
-Ask him a lot of questions about what feels good once you're inside of him.
-Jack him off, or have him rub himself while you're giving the massage
If he doesn't like it, or doesn't want it don't worry about it. Some men find it difficult to maintain arousal while being in such a vulnerable position. Some men simply don't enjoy the sensations. I've come across men absolutely who love it, and men who absolutely hate it. It's really just a person preference kinda thing.