Simple, really, you have allowed strong religious beliefs to weaken the sexual response - why on earth would he allow himself to continually get aroused knowing full well that it is never going to lead to intercourse?
Question the impact that your religious beliefs are having, and whether there is a compromise response you can make.
Not being married and having sex in a loving long term relationship is not a sin in my view - that is very different to "sleepinig around casually" with just anyone that comes allong.
As a male, just reading your post, in all honesty, killed off my sexual energy - you have to understand that men are like locomotives - full of sexual stream that needs to keep simmering away, ready for action. If the boiler goes unattended, the fire goes out and the locomotive grinds to a halt.
Your boyfriend sounds as though he has pulled up into a siging, and his engine is going out, cooling slowly but surely.
lighten up, be great friends first and foremost and talk - yes TALK to him and tell him how you feel and try and find a soultion that keeps in balance your religious and sexual beliefs.