Weird question this morning! My fiance and I have a close friend who is marrying us - he was ordained last summer. I've known him for a few years, and my fiance has known him since they were eight years old. We've always just called him by his, well, name - Kevin (haha which is not his real name - but you know what I mean). Lately, when I call him, he'll call me back and say, "Hey, katetw, this is Father Kevin," instead of just, "Hey, katetw, it's Kevin". 
So am I supposed to refer to him as Father now, even in informal situations (like inviting him over for drinks and Xbox, like we did when he was in seminary and before that)? If I ask him he will get embarrassed and tell me to call him just Kevin, even if that's incorrect.
Thanks - I know this is random!