I got an idea... some friends and I did this with laxitives. Setup a video camera and make like a cooking show thing. Act like your making some normal brownies, but slip some BBQ sauce in (in my case, it was laxitives). Make sure you do this at the end so it is after he has eaten the brownie (if he didn't taste it to begin with).
You could make it like your mom made em and have the guy over to hangout or somethin and ask if he wants food and say you got some brownies. As he eats if (if he doesn't realize the BBQ taste), pop in the video as if it were a movie and show him what he's really eating.
I did it with laxitives and it was hilarious... the guy crapped his brains out for hours.
EDIT: MrZucchiniHead, you are a sick sick man.