Right, so this is a new issue, not a precedent set by last year.
I cannot see that you have answered the following:
If your DP will be at his church of employment, why can't you go to church (at least one service, there are loads I know, sometimes!)?
Do you normally stay with him when you visit and if so, why can't you stay now? Or would you prefer to stay elsewhere/normally you stay elsewhere?
Is he volunteering/working at a non-religious event (including e.g. Christmas dinner on church premises) or is it all church services? Does it go on for more than one day (e.g. a dinner on Boxing Day, services on Christmas Eve)?
And finally what will happen if you say "Oh, I would love to see you over Christmas. What do you think - should I come to your church, or meet up on Christmas Eve/Boxing Day, or both?"
Do NOT say "Oh, X wants to know if I can spend Christmas with them but I didn't know what you were doing so I thought I'd better ask you first" because that will mean, to him, either "I'd rather go to X but wanted to check you wouldn't be offended" or "X invited you too but I'm checking you really aren't free first".