i'm in kind of the opposite situation to you, i was a complete antitheist/atheist when i started dating a Christian guy a year ago and, like you and your boyfriend, we think we're right for each other and we want to get married. obviously, as everyone has pointed out above, that's kind of not acceptable according to the Bible.
i thought it was going to be a massive problem when i found out about the unequally yoked thing but i've been going to church with my boyfriend, and we've been studying the Bible together and praying together, and i have gradually been coming to belief in the greatness of God and Jesus.
maybe God brought you together with this guy, who's got some confused ideas about what he believes, so that you could show him the right path? i don't think you should leave him because of his not having sorted out what he beleives yet. my boyfriend put up with me, helped me and showed me the right way to live, like a loving Christian should. Christianity seems to me to be about spreading the word of Jesus ('go, and make disciples' etc), maybe God is giving you the chance to lead a messed-up guy to salvation through showing him Jesus?