Mister Papa Gordon Mort Lastname, FIL

I am stuck in a long term dilemma that occasionally causes me severe social awkwardness: I don't know what to call my father-in-law. He has never given me any kind of indication of what to call him, in fact I think he's enjoying my distress as some kind of social experiment. He's funny that way.

The problem arises from the following facts:
- I have always called Paul's Mom "Momma"; all of our friends do and it feels perfectly natural
- within our group of friends we call Paul's Dad "Papa" but never directly to him, only when referring to "Momma and Papa Lastname"
- when they don't call him "Dad" the rest of the family calls him "Mort", which is some kind of inside joke that I was not a part of so I don't feel comfortable calling him that
- I will not call him "Dad" unless I am told that I can, likewise for using the first name (Gord) of anyone close to my parent's age (even if Paul's brother-in-law does)

I thought I would wait until he sent me an email to see what signature he used, but he didn't sign it. When Paul's parents give us gifts Momma always signs the cards "Mom and Dad". Last year after extreme awkwardness I obliquely asked him what I should call him (in fact I think I said, "I still don't know what to call you!" and flapped my arms in distress) and he laughed and completely avoided answering.

I'm getting really tired of saying things like, "Please pass this to your Dad." and "Where has ..he...gone." I'm toying with the idea of turning the tables on him and calling him Mr. LastName or FIL (short for father-in-law) until he gets tired of the game and just tells me what to call him, already, instead of Mister Papa Gordon Mort Lastname, FIL.