I can imagine two possibilities:
-
He has always disbelieved and known it, but thought he would get further with you via deceit. Obviously unacceptable.
-
He may just be consciously accepting what he has felt deep down his whole life, but never allowed to come to waking fruition, until now. No deceit, just growth in a direction that may or may not suit you.
I was the latter. It took me many, many years to be able to admit out loud that I didn't believe in God. I was raised in a bubble where that was simply unthinkable, and had been beaten (welts and broken skin) for once innocently saying I wasn't sure God was real, and on another occasion saying I didn't want to go to church.
Those two expressed thoughts only happened once each, around the age of 8, and then I simply buried all thoughts of those things... too dangerous to think, let alone speak out loud. So I did the rituals, spoke all the right words, prayed when expected, and for all the world looked, acted, and spoke like a true believer.
Except I never did believe, but it took a long time for that understanding to feel safe enough to resurface into my awareness, and break through the layers of protective fear and shame I had folded over it.
I don't know where your bf is coming from, it may be nothing like what I am describing, but I thought I may as well highlight the possibility based on my experience.