Just be direct and tell him as much as you love him, this is an experience you don't wish to share. I wouldn't get into your belief system (or lack thereof) with him since we generally know how that turns out.
Having had a religious mother in law who grew up in Ireland, I also think that the reason she was so religious was a coping mechanism for her shitty life (orphaned by age 9, lived with sister and took care of 11 children!); the hope that she would receive something in an afterlife was what kept her going. She also never had an original thought and Catholicism made sure that never happened.

It's yer thang! Just kidding! Respect his sincere intentions but make it clear that even if you would attend church it is b/c you love him & not b/c you think his position has merit.
I empathize with your position b/c it can be an emotional manipulation.

I never told my parents I was Atheist but I think they knew. I didn't want to hurt their feelings. Now that they are gone I wish I had been truthful with them.

Just say that you don't want to go to church because it isn't your thing. I wouldn't bother to go into much detail even if he asks because there's no purpose. Let him have his belief and you enjoy your non belief.