Thanks for the advice everyone. Let me clarify a few things.
We do have caller ID, but his parents use a cell phone for long distance and it shows up as unavailable. Also, sometimes I answer the phone upstairs and the box is downstairs.
I have had him tell his parents when he was out of town, but they will end up calling me and wanting to know what he's doing. Then, I'm stuck perpetuating his lie. Plus, this isn't really practical for his weekend trips when he's only gone for two nights.
With the current military situation, there is the small possibility that he could get called to active duty. Then he would be forced into telling his parents. He has talked about telling them b/c of this, but something keeps getting in the way. Yesterday was just the last straw for me. Feeling sick over just talking to my FIL is normal (long story) but I don't want to feel that sick.
Thanks for the advice. I will really think about it when I approach him tonight.
AP