ok then go ahead and report it because to others - it makes you look like "MOMMY! MOMMY! HE HIT ME!".
this can be settled between you guys with or without your supervisor and all it takes is a right attitude. somebody's just gotta take a high road to resolve it and then everybody's happy. if he's being a jerk and you're being a hardass self-righteous person... well things are not going to end quite pretty and that will create uncomfortable atmosphere. and because of that - boss will have to step in and fire someone which could be you. why? does it matter? 2 words - work politic.
going behind a boss's back is just a big no-no in any book (unless he's willfully engaging in wrongdoings).
but hey - that's how I deal with it. I guess we'll wait and see what other ADers say about their experiences with this similar incident.