I would just have the player remove a number of hit points equal to the mean (rounded down) for each HD lost plus Con. A wizard would lose 2 hp + Con while a fighter would lose 5 hp + Con. Feats like toughness, improved toughness and similar may come into play as well, of course.
I give all my PC's maximum hit points at each level, so this doesn't really matter.
I do know that my 1st level players absolutely HATE losing that constitution point; of course, they can always get it back when they gain an ability point later, but it's still a major bummer.