i don't know how "hardcore" of an atheist he is (probably less than me due to the fact that he's even dating a christian to begin with) but most of the time people do not take kindly to proselytizing. how would you feel if he tried to convert you to atheism? would you be open to it? would there be any chance that you would switch beliefs? it's not as if atheists are going from "nothing" to "something" so they might become believers. being an atheist often times means belief in other things that religion directly contradicts.
im one of those atheists that is
about religion, and for me it's just a dealbreaker because i wouldn't be able to take it seriously. religion is a core belief system for most people, and it is absolutely bound to come up in the long run. it's easy to look at right now and say you'd be able to overlook it, but when you're ten years down the line and all the typical issues have cropped up and started to chip away at your relationship, you're going to say "i knew from the start it wasn't going to work with our religious differences and now i'm stuck."
if it were me i would sit down and have a very serious discussion about the issue, laying out my plans and lifestyle choices as it relates to religion and having him do the same. if he is open to your demands and you are open to his, at least you guys have a shot. it's better than pretending the religion issue won't be a factor and thinking "well i can just overlook it" like it won't matter. that way you guys will be able to say you did all that you could. people with different faiths can make it work (or i've seen a true life episode to that effect. but i think they just had different religions, not religion vs. no religion.)
good luck!