Here is a wacky idea...someone who posted here said your FIL might be doing it for the attention(sorry, forgot the name).

Treat him like a teenager who acts bad to get attention. Just say, "Oh," and carry on whatever you are doing.

It's the only attention he can get...I am willing to bet that when he doesn't get attention(even bad attention is better than none, and he feels he has WON when you react), he will eventually stop.

Just a thought.

hugs
Catty1