I will tell you how I got interested in it, so maybe you can get him interested in it in the same way. Of course everyone is different, but just like your boyfriend I am a straight male who was never interested in prostate massage in the past.
The thing that changed my mind is that I read that the prostate is the equivalent of a woman's g-spot. Ever since I learned this, I have desired a woman to stimulate my prostate just as much as I have always desired to stimulate her g-spot. I was never interested in receiving anal play before, but this made me realize that it is merely a way for a woman to access another area of my genitals that I was missing out on stimulating, just as I would never deprive a woman of g-spot play.
I'm sure it would help if you explain how the prostate is analogous to the g-spot. Just as a woman who goes through life without a good g-spot massage is most likely missing out on a great experience of her genitals ability to bring her an orgasm, a man who does without prostate massage is also missing out on a great deal of enjoyment of his own genitals potential to bring him pleasure. I'm sure these things take practice and do not always automatically happen the first time.
The books I read that inspired me into this way of thinking are:
Female Ejaculation by Somraj Pokras and Jeffre Talltrees- http://www.amazon.com/dp/1569756791/ref=rdr_ext_sb_ti_sims_1
Male Multiple Orgasm by Somraj Pokras-
http://www.amazon.com/Male-Multiple-Orgasm-Techniques-Guarantee/dp/1569756252/ref=pd_sim_b_24?ie=UTF8&refRID=1CNZTTMCKXC5W5E9TB1C
I bought the two books above out of curiosity, to learn ways to take sex to a new level. I already knew I had multiple orgasms once in a while, and I wanted to figure out how to master my ability to do it. I was already aware that women can ejaculate, and I wanted to learn how to help more women get in touch with their ability to do that. It was only after I read the information about the prostate that I also gained an interest in prostate massage. Perhaps you can get these books for him and just ask him what he thinks of what he has read, then ask him what he thinks about the stuff it says regarding the prostate.
I also read another book before trying any anal experience, and it made me feel more comfortable, reassured me that this sort of thing is much simpler than it seems, and explained how to make sure that it remains enjoyable, clean, and safe:
Anal Pleasure and Health by Jack Morin
http://www.amazon.com/Anal-Pleasure-Health-Guide-Couples/dp/0940208377/ref=pd_sim_b_1?ie=UTF8&refRID=0NCPTW7YK5YY84QTDEJW
Since you are on this site and interested in this topic, I am sure you probably know everything I tell you. I just felt I would include what works best for me to get ready. That way you can help him get interested with the smoothest possible transition into the activity, since the only thing that kept me from trying it before then was that I lacked information and did not know how I was supposed to prepare myself before the activity.
Personally I think all of this is most comfortable when I have taken a shower and washed and shaved my anal area and had a few enemas until the water comes out clear and I feel everything has drained. It's worth looking up the proper way to shave that area, to avoid irritation. Normal body wash/ soap can be irritating after a shave, so I use unscented castile soap instead of body wash. As for enemas I just use water, so I connect a steel enema nozzle ( http://www.amazon.com/Cleanstream-Wide-Shower-Enema-Nozzle/dp/B007KL44PO/ref=pd_sim_hpc_3?ie=UTF8&refRID=14XYJY0XS9VW6AD1VSPQ ) to the shower hose and use extremely low, gentle water pressure that feels neither hot nor cold as it runs over the inside of my wrist. The wrong temperature and too much water pressure is supposedly very dangerous. It is also supposedly unsafe to use too much water, so once I get the water flowing just right I use a stopwatch to see how long it takes for the water to fill a 16 oz. water bottle, and after that I lube the nozzle and my anus and insert just the tip of the nozzle for the amount of time it took it to fill one water bottle, then expel on the toilet, then go back to the shower and fill up for the amount of time it takes to fill between 2 and 4 water bottles, depending on comfort, hold it in a few minutes, expel on the toilet, and do it again until all the water comes out clear. I figured I would add this info on enemas because if he has never done prostate massage he has probably never done enemas.
By the way, I always use a KY water based lube that does not contain glycerin. I think the lube is necessary for the enjoyment of the massage, and lube is most likely far more convenient and comfortable than if you just lick your finger.