Wow! I don't know if I've heard that one yet haha. I'd be willing to bet he learned it off of another kid at school. Its crazy what kids can learn off of each other. I guess to start I personally would have a long serious talk with him about why its not okay, and its not healthy to pee your pants. Its not okay because he is a big boy, and big boys just do not do that, and also because its very mean to hurt someone's feelings on purpose, and it can make his clothes smell bad too. Its not healthy because its bad for your skin and can give you a rash, its gross and when he does it, it will make him all dirty. I would tell him that you should never do gross things to yourself on purpose. Other things could easily be added to this talk as well, use your imagination and the familiarity you have with him that I don't haha.
After I got that through to him, I wouldn't acknoweledge his threads to me with anything more than "That's yucky, and it would hurt mommy's feelings, but if you're going to do it anyway you're going to clean yourself up". If he actually went through with it, I would have him take his own clothes off and wash himself off in the bath tub etc and put clean clothes on by himself afterwards, and then when he was done with that I would probably have him help me clean his dirty laundry. If he got any on the floor, I would have him wipe it up or help me scrub it up. Basically I just wouldn't let him get away with it without taking care of it himself afterwards. I think its fair enough. I'm all for helping kids through accidents, but if they do it on purpose then its a different story. And if you keep up with it, I'm sure he'll get tired of all the work involved with peeing your pants to get back at mom pretty quickly.