A couple of practical questions. How much of a “guy’s guy” is your boyfriend? What type of Catholic community do you have, uf any, where you live? It might be helpful if you can get him to hang out with other strong successful Catholic guys who practise chastity, and are none the worse for wear. It’s unfortunate that, generally, it seems to be women who have to take the lead in this area. It’s too bad that more men don’t uphold chastity for what it is - a display of manly strength, rather than a emasculating bowing to an arbitrary and pointless rule.
Regardless of your circumstances, it’d be helpful to emphasize the strength aspect of practising chastity. Practising chastity makes one more masculine, not less. Womanly admiration is a powerful motivator too (speaking as a 25yr old guy here). So make no secret of how much you appreciate the strength that he is demonstrating by abstaining. Chastity isn’t exactly common, so it really sets a man apart. Emphasize this if you can.