Thats a pretty bad joke. I don't think you're trying to "play a joke".. I think he's planning on leaving you (or you're breaking up) and you're trying to keep him around. Bad idea.
If you want to play this game, pee on the stick. After it says negative, pop open the test and make another pink line (using a pink pen or whatever) next to it to resemble the two pregnancy lines. Then close it back up.